Be careful to separate money and wealth. Rich can refer to both concepts.
It’s easy to make more money. You can literally print more of it. But I assume the question you are asking is how does wealth increase per capita over time? (Or maybe not? If you want to know how money can increase or decrease we can talk about that too)
The concept of wealth is a little tricky because we most commonly measure it in terms of money. One classic illustrative example is if two men are on an island and one is really good at finding coconuts and one is really good at finding water, they both benefit when they exchange coconuts for water (trade). Also if one guy develops a coconut harvesting device now he becomes considerably wealthier as he can get more coconuts per day (technological advancement).
Note that population growth doesn’t directly improve per capita wealth. That said perhaps you need at least 10 people working on food and shelter before you can have someone focus on making art. And the larger your population the more exceptions individuals you have who might benefit your entire society.
There’s an analogy to be made here about demand, capital, technology, and economic growth that’s sort of fun. But hopefully This makes some sense. If you are really interested take Econ 101. Badecomics subreddit is also a fun place to read what actual economists think about various economic topics. Igm forums are a fun and super accessible place to see what actual economists think of various topics. Then if you want to repeatedly smash your forehead into your desk read your average reddit thread on anything economics related.
Let's ignore money for the time being and look at creating value:
If I plant a tree, let it grow, chop it down and make some furniture out of the wood, I have created something of value which didn't exist before. I have thus got a little bit richer. I could have used other examples eg taking clay from my garden and making a pot.
It gets even better if two people create value as they can trade with each other. Maybe my tree furnishes enough wood to make ten chairs, but I only need 2. I can swap those 8 chairs with other people who also have an excess of whatever goods they manufacture. Each swap makes both participants richer. I would rather have the other goods than the chairs; they would rather have the chairs than the goods they swapped.
Now money, is really just a way of facilitating the swaps. I might have a chair I want to swap, but nobody who wants a chair has anything I want. But if I can swap the chair with person A for some money, and then I can swap that money with person B, for whatever it is I want, I am happy - and so are A, who by giving me some money has completed his swap, and B, who is now able to swap the money with person C for whatever he wants).
So the value does not lie in the money itself; the money just represents the value that has already been created. The money each individual possesses at any moment represents the value they have created for other people minus the value others have so far created for them (for which the individual has paid money).
As such, you don't need extra money to represent the extra value. The extra value lies in the objects and structures we have created.
Money is just a representation of value creation and value is constantly created. For example someone could buy a $2 notebook and $1 pen and create a $1.000.000 screenplay.
We make it, most everyone one of us. We take raw resources, add effort, and out comes wealth. Note that money and wealth are not the same things. Wealth is anything of value, money is a tool we use to assign value to and exchange wealth.
If I make a tomato plant grow where it wouldn't have otherwise, I get tomatoes I can eat. That is a thing of value that didn't exist before, and the world is slightly richer now. If I grow a lot of them, I can trade them for the coal you dug from your field or the fish you caught from the river. You didn't make either one, but you changed them from an unusable to a usable form, increasing their value.
You might take my tomatoes and can them, adding even more value to them. I might take your coal and use it to turn rocks into metal. The coal is lost but replaced by something of greater value. In all these cases, something exists that didn't exist before, or exists in a form that makes it more valuable.
The same is true for services. If I clean your house for you, that gives you more time to dig coal and catch fish, indirectly creating more value. If you teach me how to make fertilizer out of your fish, I can grow more tomatoes, also creating more value. Any time you pay someone to do something for you, it saves your time, money, or both, which you can use to creates something else of value.
Money is representative of work done. It’s traded for goods and services. Everything can be broken down into individual parts. A 20,000 dollar car is representative of individual nuts bolts, glass and plastic. When you spend money on it you pay for those individual pieces, every technician who put to together and built it, every person who worked on it. Some of those resources are recycled, but most are pulled out of the ground. The iron and aluminum from mines, silicone dioxide for the glass from beaches.
The car you bought sticks around for a few years. Once it’s built and paid for you can devote your time and effort buying other things. As long as it lasts longer than it takes to pull a new one out of the ground that wealth continues to build up. You can pay for another and it’s pulled out of the ground, assembled and built and delivered to your door.
Some of it is simplified with robotics and automation, which was itself pulled from the ground.
The same idea can be simplified. If you have nothing, and you go into the Woods Minecraft style and build a stone axe and use it to build a wood hut, the wealth is coming from the resources around you.
Most of them are renewable (wood) because of the sun, some have nearly limitless supply, (iron) because of the size of the planet. Some are more limited, like oil (how limited is another discussion).
So when you ask a 5 year old where he got his diamond pick axe, he made it himself.
TLDR the wealth is pulled out of the ground, forged into useful items, by us.
productivity gains. It used to take thousands of people to farm the same amount of wheat that a handful can do today, for example. Every day someone somewhere is figuring out how to produce more with less manpower and materials.
In a perfect world, this is possible if everyone does more work. In this fair world, money is a representation of someone's labor. The foods we eat, things we buy, and services we receive are available because someone worked to create it. So let's suppose everyone worked more, then there would be more goods and services to go around and everyone would have more money to buy these things. Of course, there are only so many hours in a day to work so that puts a limit on how much wealthier this makes us.
Another way to make everyone wealthier is by making everyone's work more efficient. This is possible with advances in technology. If you think about all the food, products, and services available to an average American in 2018 and compare that to one in 1818, then it is fair to say people today own more wealth because there are machines to make our jobs more productive. People used to work longer hours to create fewer goods. But even today people in some parts of the world don't have access to technology and its benefits.
In the real world, not everyone gets richer. It's hard to tell how much work of one type is equivalent to another type, so it's up to people's idea of its worth to give money its value. Often what happens is a small group of people gets richer by having ways to imbalance the exchange of money in their favor. That is, by force or by persuasion they make a large group of people do lots of work for less in return. A person in this small group could see everyone in this circle getting richer, without realizing there are those on the outside creating that extra wealth without being fairly paid.
Also, money itself has no value. Hyper-inflation in countries where the government is collapsing can make everyone have lots of money, but the money can't buy much at all, so rather than getting richer, these people are getting poorer.
Wealth is merely what we produce, with robots and more education we are becoming more productive and thus produce more per capita and thus wealth is created.
The money is a measurement of the wealth that has grown. The growth in wealth results from increases in human productivity (creating more with less effort), due to advances in technology, increasing human knowledge, more physical objects to assist us.
More overall money is just a representation of specialization and better technology. If it used to take 30 hours to build a chair, and technology allows us to make 30 chairs an hour, that extra value is in the cost reduction.
As technologies improve, there's more excess to go around. This tends to effect necessary goods first. (food and water became food and water and shelter, became food and water and shelter and plumbing and medication, became food and water and shelter and medication and education and insurance and etc etc.)
As you focus the efficiency of the resources spent, potentially everyone gets better off. I say potentially because many people hoard or always want more, which isn't bad, but it's not exactly the common good in some situations.
So look at your necessary time to have the abject minimum, a few thousand years ago you could spend a lot of time just trying to stay warm. Now you want an Xbox. Both require hours of effort, but now instead of having to spend 8 hours hunting or foraging, only half of that is for necessities, the rest is for luxuries and convenience.
Tldr. You only have 24 hours a day, think of this as 24 points. Food and water alone used to cost 8 of those.
As technology and partnerships/trade increase, food may net only 3 of those points instead. Maybe even less. Those extra points are how everyone gets wealthier.
This example assumes people will be efficient with their time and not live beyond their necessities. I know that's a silly thing to assume, but preferences skew the model at its base.
The Earth has limited resources. Economic growth comes from people finding out how to use those limited resources more efficiently. This happens in a few ways.
1. Innovation. A farmer has to manually plow a field. The field they can plow is only a 4 acres big by hand. Now one farmer can plow 40 acres resulting in 10 times more food. Now they can use machines to plow 4000 acres so there is 1000 times more food. Now there are pesticides, fertilizers, GMOs, etc. Now one farmer can handle 40,000 acres resulting in 10,000 times more food.
2. Specialization. Three people are farmers/bakers/cheese makers. They are jacks of all trades, but masters of none. So one person focuses on farming, another takes the wheat and makes bread, and another take the milk and makes cheese. By focusing, they can be 10 times as efficient as before. Then a fourth person can come along and become a chef who combines cheese and bread to make grilled cheese.
3. Utilization. There are some resources on Earth that are very rare, like gold. Others are very common, like oxygen. There are some resources where there aren't enough people alive to make full use of them. For example, in the above example, if there are only 3 people in your town, there won't be a fourth person to become a chef. So population growth has made people more money because fewer resources are going untapped. This a problem for resources used unsustainably, but is otherwise good.
The extra *wealth* comes from increased production (i.e. goods, services, etc).
The amount of *money* in the economy is not necessarily related to the amount of *wealth* in the economy or how "rich" everyone is.
A common representation of economics is "pieces of pie" or a "zero-sum" game. The notion is that there is a certain amount of wealth in the world, and it is divided (unevenly) among all people. If if someone gains more, it must have come from someone else. If I get $100, someone else necessarily lost $100. This model would tell us that everyone *can't* get richer, and that the extra money can only com by taking it from someone less fortunate.
Luckily, none of that is actually true. In reality, wealth can be *generated* - we aren't limited to slices of one pie, we can just *bake more pies*. We can invent new flavors of pies. We can come up with food that isn't pie, or even pie shaped, at all!
You see, money is just a physical representation of the concept of *value*. We can get more money by simply increasing value. And increasing value is, actually, quite easy.
Consider this: in mathematics, we call the result of addition the *sum* and we call the result of multiplication the *product*. Now consider a factory, what does it do? It *produces* things, it doesn't *sum* them. A *product* is more than the sum of its parts, it has a greater value than the stuff you put into it would have on their own.
A "zero-sum" game makes sense when you can only add and subtract - it says that for every addition there must be an equivalent subtraction. But when you throw multiplication into the mix, it all breaks down. The numbers can get bigger anyway.
And that's why "everyone is slowly getting richer" - it's because as we advance we can produce more value, and we represent that value with "money." The extra money (value) doesn't "come from" some place, it exists as a byproduct of production.
Consider the modern cell phone. Phones only cost a fraction of what they are sold for. The difference between the cost and the price is "profit" and it isn't a bad thing. Profit is that "extra money", it's the value that was created above and beyond the value of the materials that went into the phone. A lot of people wonder why this is okay, they think that selling the device for so much more than it cost to make is wrong, or unfair. But in reality, the price isn't about what it cost to make, it's about the *value* of the product. And the value is defined by people: what are they willing to pay?
Any time people will be willing to pay more, the product has more value. Even things with no practical purpose - like a designer watch that doesn't tell time any better than a cheap one - is more valuable because there is something about it that people want and care about. Something they *value*.
If I gain $100, it doesn't mean someone lost $100 (unless I'm a theif, or a con-man), it means they exchanged it for something that, at least to them, has a value equal to $100. Since money is just a way to represent value, the fact that they no longer have $100 isn't important, they still posses the same quantity of value they did before. But in another form. If they want more, thy can produce more. If not by their own hands directly, they can sell their time (employment) in exchange for value (money) they assisted in the production of.
As long as we can keep increasing the value of things, we can increase the amount of wealth that exists in the world. We can just keep baking more pies, and pizzas, a quiche, and anything else we can think of.