general concepts applicable to other tooling/languages: parser/printer/compiler/AST/tokens/etc (thinking https://regexone.com to learn regex, or same with HTML/CSS/SQL/etc)
specific things in implementation of babel (aka how to write plugins, many examples of how to do x with examples), use babel-handbook, use code we’ve written in babel plugins or 3rd party ones.
Teach how babel works interactively