Copyright really is not only concerned with what exactly is on the page, but also how you got there, and where the knowledge came from to get you there.
What if I read your codebase, and then years later while programming for myself I inadvertently use solutions you came up with while thinking I came up with it myself?
There really are no hard set rules, and this is something that is handled on a case-by-case basis based on whether or not a convincing argument can be made that you copied a novel idea from someone else and claimed it as your own.
We can argue the semantics of it all we want, but the subject area is an active battleground. Typically it only matters when money starts to get involved, since no one usually presses the issue or gets involved with random personal projects. So when an enterprise level company leverages that lack of caring into a proprietary pay-to-use project that operates by copying and pasting code from copyrighted material, then it seems like a case might be able to be made for it.
There are no rules about the form of the code itself that governs whether or not someone owns it. Common sense applies. Sure you could "steal" very small, common, code snippets and get away with it; but that doesnt make it less wrong.
When a commercial entity explicitly does it, however, some times we can catch them. Like if they do it through algorithms that we more or less know how they work - i.e. the algorithm is using advanced control flow logic to copy and paste from it's training data set and copyrighted material is in that data set