Start reading the source code.
For “advanced” anything, you have to go to the source.
For “advanced” anything, you have to go to the source.
For example, you can read all of the Linux source code to become expert in most areas: networking, scheduling,... (except AI/ML) But difficult to achieve without a good structure or you can read a book about Linux kernel development (e.g. Linux Kernel development, old book by Robert Love) and get more up to speed