It's hard to explain all of this in a comment, but here are some key points:

- All bitcoins that exist can be audited. The blockchain ist public and mirrored on tens of thousands of nodes.

- The number 21 million is the result of a formula in the bitcoin source code. About every 4 years the amount of new bitcoins is reduced by half... until it reaches 0 in the year 2140. Currently 6.25 Bitcoins are created every 10 minutes. From April 2024 it will only be 3.125.

- You are right, since this formula is in the source anyone could change the number of coins by changing the code. The problem is, that those tens of thousands of nodes running the bitcoin software wouldn't accept these changes, unless you could convince a majority of users to run that new software. So while it is technically possible to change the rules, no one would agree to run this "watered down" version of bitcoin.

- In fact the code of bitcoin has been copied and changed several times already. Those projects are known as Bitcoin Cash, Bitcoin SV, ... and don't have much support.