Real world OS is without no doubt complicated. But for beginner, I think xv6 is sufficient: https://github.com/mit-pdos/xv6-public https://github.com/mit-pdos/

Real world OS is without no doubt complicated. But for beginner, I think xv6 is sufficient: https://github.com/mit-pdos/xv6-public https://github.com/mit-pdos/xv6-public A rather small codebase, pretty well documented, easily buildable without building your own custom GCC, docker etc etc