[sosuke]

I agree with you; getting that out of the way.

But if I have BTC, there is a specific BTC that is mine. That is how I've understood it. The IRS tax guide for crypto says as much talking about buying and selling two coins bought from two different years.

[thesausageking]

That's not how the code works. Wallets have an amount of Bitcoin, but there are no unique identifiers to a Bitcoin. They don't have serial numbers like, say, a US dollar. There is only the transaction history that shows an amount being sent/received between wallets.

But even if Bitcoins had serial numbers, they would still be practically fungible. Dollar bills have serial numbers and 99.99% of the time, dollars are treated as being completely interchangeable (ignoring the corner case of people hanging the first dollar they made on the wall, etc.). If you give a store one, they treat it as if you gave them $1 in value and don't care which specific dollar you gave them.