Hacker News Reader

Well, yes. Kinda. You need to know a bit about high level organization and structure of OSes first as the codebase is not particularly small.

For FreeBSD specifically there's a book about it that is written by the very people who designed most of it: https://dl.acm.org/doi/abs/10.5555/2659919

Pretty much, and also there is more documentation for some parts as linux. Not the prettiest code in the world, but ok.

Quite, IMO.