How do I Resolve BTC Gifts with the IRS?

Trying to understand tax implications (if any) for BTC gifts or "anonymised" wallets.

Let's say I received 1 BTC from someone who bought using an exchange at 40K USD. Do I pay taxes on that BTC as a gift of 40K?

Further down the line after running it through mixers and such, I decide to sell it at 30K from an exchange, and I assume the exchange reports this transaction to the IRS. This taxable event is triggered but how does the IRS know it's considered a loss? And moreover, if I didn't have the knowledge of what the gift was valued at, how would I know to report it as realized gains/losses?

Thanks for any and all info.