UNIX is basically a simple operating system, but you have to be a genius to understand the simplicity. - Dennis Ritchie

I also find the high-level design of operating systems pretty interesting. This started with this excellent course from Georgia Tech.

Even if you don’t really care about operating systems, I recommend this particular course on the same grounds as I recommend Nand 2 Tetris: it’s about more than the technical details. There are patterns at work that apply across disciplines.