How does Emacs load a non-existent .el file?

[Konstantin Kharlamov <Hi-Angel@yandex.ru>]

I was just looking whether it's too hard to add an option for reading
from stdin (a very requested feature¹). Long story short, I presume
command line parsing happens in `lisp/startup.el`, function `(command-
line-1)`. That's where the weirdness starts. Since it is an ELisp file,
I figured to avoid recompiling I can move its `.elc` file out of the
way and just do edits to `startup.el` directly. Turned out though, not
only Emacs does not notice these edits, it somehow manages to load the
file when it does not exist!

So e.g. I did a `sudo rm /usr/share/emacs/30.0.50/lisp/startup*`, so no
.el or elc files. But starting up `emacs -Q` and asking it `C-h f
command-line-1` still results in Emacs answering that such function
exists, except the help buffer doesn't have a link to it.

I am thoroughly confused. Any idea what's going on here?

1:
https://superuser.com/questions/31404/how-to-make-emacs-read-buffer-from-stdin-on-start