How exactly is capital gains tax calculated on crypto gains? I have an example.

Let's say hypothetically, I bought certain coins on a SOL wallet that doesn't require any personal identifiable information to set up.

I funded this wallet by sending USDC from a different wallet, the bought other coins with the USDC.

If hypothetically speaking I ended up making a large sum of money by converting gains to USDC, how will it work if I later decided to sell this USDC for fiat and send into a bank account?

Who will come knocking and what questions will I have to answer?

If hypothetically speaking I made 5M from a 10k investment, how exactly will anyone know this unless I tell them? What if I say I started with 2.5M already?