This is in general a good advise, but not a practical at a times. For example, you can read all of the Linux source code to become expert in most areas: networ

This is in general a good advise, but not a practical at a times. 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