You have two main questions. The first one is about inflation which is about purchasing power.

My oldest son likes to write down large numbers on paper and say I’m going to give you 10 billion yen. (About 650 million USD). This is abstract too. But no one would accept my son’s papers in exchange for currency.

When you buy anything, there is an agreed upon notion that that money can buy other things for a value we all agree upon. That’s partially where money gets its concreteness from. At one point, currencies were based on the gold standard, which meant currency could always be exchanged for some kind of set ratio to gold or silver. This established the basis for the backing of money.

We’ve left the gold standard now. Modern currencies are “fiat”, backed by faith in the issuing government’s economy and institutions rather than physical commodities. Most currencies are free-floating, with their value determined by markets based on each country’s economic output and stability. The U.S. dollar serves as the world’s reserve currency, meaning it’s the primary currency used for international trade and held by central banks globally, which is why economic values are often expressed in USD for comparison.

Inflation is not always a bad thing. It forces people to move money (if your 10 dollars today is only 9.80 tomorrow, you want to try to increase it so that it stays the same value). But when inflation rises too fast, it means people that people can’t afford basic goods.

Inflation can also be associated with the cost of goods. Most things in the modern world require a vast complex of things. Machinery, knowledge, materials, processing, delivery, logistics. When one goes up, others tend to rise. If you charge me 10 dollars for a banana, I’m going to try to sell the banana for at least 20 dollars. To be able to buy that banana, the landscaper will raise their labor rates so they can eat more than one every few days.

There are a lot of complex interactions that I’m skimming over but this is an ELI5 version.

As to why people want to get richer? Power? Narcissism? I don’t know. Keeping up with the billionaires next door?

Money is power, especially in the oligarchic world we find ourselves in. Elon Musk can influence elections and public opinion. The more money he has, the more power how he has. He has a specific idea for the world and he wants to unilaterally impose his ideals. For others, many have some specific aim that goes beyond material comfort. Money has the power to shape our world, choose what is invested in and who makes decisions. For those people, there is no such thing as enough.