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Some college CS departments have courses in OS development. If that's not an option, I'd start with a textbook like Operating Systems: Design & Implementation, aka the Minix book, since it documents the development of the Minix operating system.

https://wiki.minix3.org/doku.php?id=www:documentation:start

Generally you’d look to learn about OS design and reverse engineering.

For a hands-on introduction, writing a minimal kernel in Rust, see https://os.phil-opp.com/

The OSDev wiki is nice.

https://wiki.osdev.org/