Yeah, due to the friction of maintaining a private fork, I always upstream bug fixes I find where the fix is simpler than a work around. It's just neighborly. I don't it very often, maybe every 2 years? But a lot of other people do so, and it adds up; open source software continues to improve pretty substantially over the years.

And when I'm debugging I read all the code I have access to - certainly glibc pretty often, Python C implementation and C libraries quite often; the Go runtime, if only for the education, etc. It's not often I need to read the kernel implementation, except sometimes for networking protocols where it's also educational. I have found over my career that reading excellent code is as important to the quality of my code as writing code and iterating on it to make it good.