It's GNU/Linux, the part you need to learn is GNU (the userland), not Linux (the kernel). GNU is a Unix-like system, so any resource on Unix will do. There is a number of books and tutorials, some of them free. Start small with things like ls, cp, pwd, echo, touch, rm, mkdir, rmdir (don't bother memorizing those names for now). Then learn that all these "commands" are actually just the names of programs and that you can add new "commands" by installing or writing your own programs. You can write down a series of commands into a file and then run that file, it's called a shell script (because the thing that interprets your typed-in commands is called a shell).
Back when I was a Mac fag I used this resource:
developer.apple.com/library/archive/documentation/OpenSource/Conceptual/ShellScripting/Introduction/Introduction.html
There is also a popular guide written by GNU:
write.flossmanuals.net/command-line/introduction/
I haven't read it, so no idea how well it is written, but it seems to be popular. You can also buy a printed copy if you really want to from the FSF.
Both. You need to know some basics, which is what the tutorials teach you. Beyond that you can always read the manual pages (also called manpages in short). Type 'man whatever' where whatever is the name of the command, for example 'man ls' will show you the manual of the "ls" command. Be aware though that the manual is a true reference manual, it is very technical and expects you to be familiar with Unix conventions. Manpages are very terse and are usually not proper user manuals. For a large nicely written user manual you have to look up the page of the project, usually they have links to a large manual. For example, GNU Bash has a link on its page to the full manual in various formats:
gnu.org/software/bash/#documentation
In the case of GNU projects you can also often type "info whatever" to read the large manual in the terminal. It uses the standalone info reader which has retarded key bindings though, so you should instead read "info info" first. Or use another info reader. Or just get the manual in HTML or PDF format. Or buy a printed copy.
A final word on documentation, Ubuntu often does not install manuals when you install a package. For example, if you type "info bash" you will only see the manpage. To the the full manual you will have to install the package "bash-doc" (sudo apt install bash-doc). In general, if the package is named "whatever", the the documentation is in "whatever-doc".