One important thing people should keep in mind, if they're starting only with Emacs navigation features for code and docs, is that they're missing crucial information that's in the manuals. One of the manuals is like a combination of the "API docs" you'll find in the docstrings, plus all the narrative around those, which you won't find in the docstrings. If a newbie is told only to browse docstrings and code, they're doing it on hard-mode.
And if a person makes sure they have the manuals installed (sadly, the Emacs Lisp manual(s) don't come installed by default in distros I've seen, which I think is a tragic mistake for uptake), and learns how load the manual in Emacs Info browser, and learns keys the Info keys like `i`, `s`, `[`, `]`, and `u`, I think they'll be glad they didn't try working from manuals in the Web browser.
Also, I wanted to start with making the code files more accessible and concrete, and generally tell people to try thinking differently about this. The reason is that I've suspected that a systemic barrier to accessible Emacs hacking was being created in the bureaucratic and obscuring direction things were going. (And it sounded like maybe this person was a victim of that suspected problem, since, when I started, it was much easier to get into, in the ways I described.)
(FWIW, I was able to write some Emacs extensions by starting with the approach I've suggested, including: https://www.neilvandyke.org/emacs/ )
Here's a bunch of tricks one should know if one wants to learn Elisp:
1. You can use describe-function (C-h f) or describe-variable (C-h v), as well as various apropos functions (starting with apropos, C-h a), you can find documentation about pretty much everything in Emacs, down to every single internal function or variable of any bit of Elisp code.
2. When you do that, you get a help buffer that almost always starts with something like "foo is a variable defined in `foo.el`". That "foo.el" bit is a link. You can click on it (or move your point to it and press Enter), and it'll land you straight at the definition of that function/variable. This is your shortcut to jumping straight into relevant source code of everything. Including C code, if you dig yourself into Emacs internals.
3. You can navigate to the help buffer in 2 from plenty of places, including e.g. the list of current modes, or description of a mode.
4. When you're playing around with Elisp code, xref-find-definitions (M-.) will take you straight to definition of a function under point.
Emacs is thoroughly documented and cross-referenced. Your own installation is its own single, living codebase - and you can navigate through it as such.