As for more serious projects I am interested in and could be a potential follow up to contribute to a bigger kernel, there are SerenityOS and RedoxOS.
I still tinker with the x86 version (which isn't maintained anymore): https://github.com/mit-pdos/xv6-public. Doesn't need much effort to build it on macOS.
The codebase is pretty compact that it can be understood in a few days, and also rebuilding it is very quick. Can't say the same thing about Linux, but hey as real world OS, guess some complexities are simply unavoidable.