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
Start reading the source code.
For “advanced” anything, you have to go to the source.
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
Also, if you learn just by reading the source, you won't get a lot of valuable information about why certain design decisions were made.
At least you’ll have something to talk about.