I use TLPI as an optional text for my CS Operating Systems course! It's honestly the best resource for a comprehensive look at the innards of Linux. I actually even snip select pages for lectures.
People should do a foundation course to figure out which deprecated parts of the kernel source to avoid. It is nontrivial, but talking with the active developers will save a lot of guess work. =3
Introductory LFD103 is a free course:
https://training.linuxfoundation.org/training/a-beginners-gu...
Some channels to get some experience handling the modern kernel source:
https://www.youtube.com/@johannes4gnu_linux96/videos
https://www.youtube.com/@nirlichtman/videos