For me, when I learned more than 20 years ago, reading source and manpages from Linux and other Unix like projects was a source of inspiration.

I'd recommend OpenBSD libc. https://github.com/openbsd/src/tree/master/lib/libc

String functions especially are a good way to get into the C way of thinking. Since C's approach to strings is unique.

I also learned a lot by reading manpages of libc functions or Unix utilities and thinking about how they were implemented, and writing my own little versions.