> The tutorial barely explains how M-x actually works, but also doesn't mention the describe-* functions at any point.

It actually does, but not by name. It points you to the help system. In particular:

- C-h a - help apropos, search for commands by part of the name getting a list

- C-h k - describe-key

- C-h c - describe-key-briefly

- C-h f - describe-function

- C-h v - describe-variable

All of those are in the tutorial.