So Bitcoin has 21,000,000.00000000 in existence.
Bitcoin does not 21.000.000 in existence and never will have.
The actual maximum supply is below 21 million. And the amount of coins in circulation currently is at about 17.3 million (ignoring 'lost' coins).
The maximum supply is NOT hardcoded. It can be derived from the function which regulates the amount of freshly generated bitcoins by mining.
How does someone determine how many zeros are placed behind it? Could someone make an asset that has 1 coin but 1.910980182309824791827981240912094109249129837 in total token supply ?
The actual denomination is handled in the source code.
You can choose 'as many' decimal places as you want when creating/forking a coin.