Bitcoin works differently than Ethereum.

In Bitcoin there are UTXOs. You can imagine them as dollar bills.

So, if in year 1 you buy 1 BTC and send it to your wallet, there will be 1 UTXO of 1 BTC in your wallet.

If in year 2 you buy 1 BTC and send it to your wallet, there will be 1 more UTXO of 1 BTC in your wallet.

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If in year N you buy 1 BTC and send it to your wallet, there will be 1 more UTXO of 1 BTC in your wallet.

So, if in year N+1 you decide to sell, you can decide which UTXO you want to sell. You can also sell a fraction of the UTXO, or combine multiple UTXOs and sell them.

I don't know how it works with accounting but that's how it works with Bitcoin. So you know which BTC you sell at every time, provided that you have a self custody wallet with coin-control ability. If you keep your Bitcoin in exchanges, then it's a whole other story and you should WITHDRAW YOUR BITCOIN FROM EXCHANGES