You can scroll through /bin, /usr/bin/ and /usr/local/bin/ and find all the included basic tools and then read "man 'tool'" or google for it
Coreutils is a good place to start.
I spent 10 years avoiding awk, because every time I typed 'man awk' I was overwhelmed by the GNU monstrosity. Then I found the man page for the plan9 implementation of awk (http://man.cat-v.org/plan_9/1/awk) and learned the language in fifteen minutes.
Rather, info manuals are GNU's official (self-invented I think) documentation system. They make manpages only because of their popularity, but they direct you to the info pages for more info. When other projects have too much documentation for a manpage, they split it among several pages, like perl, openssl, zsh, git, borg, btrfs, etc. Personally, I like both, and wouldn't rank one above the other.
If there is a better source, I don't know it.
One notable example is the time I had to learn C# 4 years ago. It took an entire morning to scan through the entire language reference and then later I found out I already knew a lot more than "advanced" devs who would just look up stuff as they needed it, without knowing what actually is available in the first place.
I feel its the only sensible advice if you want to learn a lot on your own with limited time.