A computer is a highly layered system, and so you have to decide which layers you are the most interested in studying.
It must also be said that like in any industry, certain layers are hidden in commercial secrecy mysteries making it harder to actually learn them.
But as you climb down into the abyss of low level hardcoreness, don't forget that making usefulness is more important than being hardcore: Figure 66. "xkcd 378: Real Programmers.".
Summary from low-level to high-level:
