OSDev can be hit or miss. All of the bootstrapping was done following this [0] tutorial for Rust, which translates easily to zig. For the more advanced parts you can find usefull ones in [1] or use the OSDev search bar. For even more advanced topics though you'll find that there are no tutorial and only a few open source implementations to take inspiration from!

ps: I've updated my readme with a few references

[0] https://os.phil-opp.com/

[1] https://wiki.osdev.org/Tutorials