Is there any particular language you're looking for? I've found some languages hideous until I understood them and could appreciate their respective graces. Off the top of my head the I can think of a couple of projects you and others may be interested in.

The first is Jones Forth (https://github.com/nornagon/jonesforth), start with jonesforth.S and move into jonesforth.f. I really enjoyed following along with it and trying my hand at making my own stack based language.

The other is Xv6, a teaching operating system from MIT (https://pdos.csail.mit.edu/6.828/2021/xv6.html), not all the code or implementations are top notch but it shows you non-optimized versions (just because they're simple and more readable) of different concepts used in OS design.

If you're interested in the embedded world, there is a really neat project I've been following that feels a more structured and safe (as in fault-tolerant) while still staying pretty simple (both conceptually and in the code itself): Hubris and Humility (https://hubris.oxide.computer/).