I wanted to make a quick note to say that whenever you encounter an issue or want to configure something, use Google to find what file in particular you are editing. I mentioned some files I deem are sort of foundational to know (such as
/etc/passwd), but I think that in 2020. it is reasonable to assume that you have access to the World Wide Web and that you can use Google to answer queries such as “How can I modify systemd startup services” or similar.
In the language of optimization, no need to over-optimize for the goal of knowing about Linux. Know just enough to be confident that you have the major pieces of the puzzle and know how they work, but if a particular detail pops up use Google to solve it.