We literally read basically all of the Linux scheduler/syscall code in my computer engineering classes in the first couple weeks, the rest of the course was to

We literally read basically all of the Linux scheduler/syscall code in my computer engineering classes in the first couple weeks, the rest of the course was to reimplement a subset of it for a custom (RT)OS on some Cortex-M3 microcontroller dev boards. The first week or so of the process was learning how to go from a new Debian machine (with the expectation we'd only used Windows or Macs before) through "git clone git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git" (not on Github at the time) to finding the function called by "man 2 fork" and navigating the source tree/unmasking preprocessor stuff to show the actual implementation. Yes, you can easily make a wrong turn if you're trying to do that on your own. But the actual linux/kernel directory, which has most of the parts you'd want, isn't that much larger, and a lot of the difference is modern requirements like power saving and security.