A company is valued in terms of the currency in it's stock exchange. A company in US has it's market cap in USD and a Japanese company has it's market cap in JPY. If you want to calculate the market cap in USD for a Japenese company, you have to use an exchange rate.

Now since cryptos are currencies themselves, it makes no sense of talking about a market cap. The market cap of stellar is the number of stellar available.

The market cap of a real currency like USD is the money supply. The money supply has probably a couple dozen different definitions depending on how you want to look at it and the fungibility of different representation of money. (Eg does it really make sense to consider burned bitcoin or satoshi's supposedly "never moving" bitcoin as part of the "money supply")

https://en.wikipedia.org/wiki/Money_supply#United_States

There is a reason forex market economists/analysts don't talk about the "market cap" of the Japanese Yen or market cap of the British pound.

There is also the whole intrinsic value of a company part. People buy stock because they believe the company is worth more than other people do. They (except for minor exceptions in the case of small investors) judge the companies future ability to generate profits and cash to decide on whether the buy or sell the stock. People buy crypto on pure speculation that other people will buy crypto in the future.