What part of a bog-standard HTTP API can be copyrighted? Parsing the POST request or processing it or shoving it to storage? I'm genuinely confused here and not just being an ass.

There are unique algorithms for things like media compression etc, I understand copyrighting those.

But for the vast majority of software, is there any realistic threat of hitting any copyrighted code that's so unique it has been copyrighted and can be determined as such? There are only so many ways you can do a specific common thing.

I kinda think of it like music, without ever hearing a specific song you might hit the same chord progressions by accident because in reality there are only so many combinations you can make with notes that sound good.