For on how to use GNU user-land tools (Bash, cat, less, grep, sed, etc.), I recommend the GNU manuals which are very well-written: www.gnu.org/manual/blurbs.html (example: Bash manual);
GNU manuals are also downloaded for offline read by default. These manuals are provided with info.
For GUI desktop, it depends on what Desktop Environment (DE) you use:
If you don’t know what DE you use, KDE uses sharp edges for windows and GNOME uses round edges ( 👁️👄👁️ ).
For system, as @Terumo pointed out, for system-specific stuff, there’s ArchWiki that has topics about everything.
If you want to understand what are the /usr, /usr/local, /etc, /dev, etc. filesystem directories, you want to look at the Filesystem Hierarchy Standard: refspecs.linuxfoundation.org/…/index.html
If you’re a developer and want to find out how deep the rabbit hole is, you may want to look at an online book Operating Systems: Three Easy Pieces: pages.cs.wisc.edu/~remzi/OSTEP/
EDIT: Forgot this important material:
If you need to know command-line argument specifics for a particular program, use manpages (For example, to find brief information about grep, type man grep in your shell, and info grep if you need a complete manual).