But why couldn't they, e.g. buy other people's thing in exchange for their gold? IOW, why is the relevant fact that _Spain_ didn't have much more wealth despite having more gold? Other people in other countries valued (and still value) gold too.

Think of difference between buying bunch of cars from someone making cars. Instead of buying the factory making cars.

Or maybe currently RAM. You found pile of money somewhere. You buy big amount of RAM. RAM prices go up, you do not care you have money. In a few years DDR6 comes out. The RAM you bought is not that expensive anymore. You do not really have either money or something currently expensive.

Wealth is owning the land, the machines, the buildings, the knowledge(more so now). Being rich is owning lot of what can buy those things. But if you keep spending liquid wealth, eventually it is gone and you are not wealthy anymore.

You can, but then those people have more money, and you have less. The money supply is much smaller than the supply of goods and services, because the same money circulates repeatedly. So simply doubling your money can tilt things a little more your way, but does not make the situation completely lopsided, because you can only spend the extra money once.

Also, to quote Jastram [0], "In spite of the romanticism of the Spanish Main and treasure ships, the supply of gold from the New World was just a trickle by later standards. Less than 1% of what was to be 1930 world production was produced in each of the years from 1500 to 1520." Over the next 80 years, it never got much above 1.3%.

[0] Roy W. Jastram, The Golden Constant (1977), pg. 41.

They could and they did. That meant that Spain ended up importing more and exporting less, which would have been bad for Spanish businesses.

There’s a lot more to “wealth” than just having a bunch of stuff. Especially in the long term, since most “stuff” will wear out or otherwise decay over time.