I notice under my /usr/include directory, there is a folder called "bits" with a bunch of header files. Some of the files are "bits/errno.h" or "bits/in.h".
Where do these files come from? They don't look to be part of libc. E.g, musl doesn't ship with them: https://github.com/cloudius-systems/musl/tree/master/include, however, I do notice that musl does depend on them.