It's one of the the great things about Open Source software is that you can learn as much as you want, there are no secrets and no hidden agendas obscuring learning.
Can one ever know everything about Linux? Linux is big, I mean really big. You just won't believe how vastly hugely mindbogglingly big it is. Seriously though with sci-fi references aside, we can talk and write a lot about Linux and ever really cover everything. Although technically it's only the kernel and as such a small part of the operating system, we all think of the entire operating system as Linux and that means all the open source programs that are part of it. It also means the architecture and technology that Linux uses and the platforms on which Linux runs.