I don't run under macOS any more, but when I did, there was an extra
wrinkle here: emacs typically builds as an "app bundle", essentially a
directory named "Emacs.app" that contains all of the stuff that a typical
unix-like installation would put in various subdirs of /usr/local. This was
actually quite nice -- it meant that I could have a complete, functional
version of emacs in both Emacs.app and Emacs-old.app whenever I wanted to
try something new without stranding myself if the new thing turned out to
be broken.

I gather that more libre distributions are attempting something
functionally similar via things like Snap and Flatpak, but I don't have
much experience with either. I bring it up here because it's likely to add
an extra wrinkle to finding source files under macOS, especially if the OS
still comes with a (quite old, tty-only) version of emacs in /usr/local
that should be ignored (MAYBE: except for site-lisp?) by modern emacs.

Hope that helps,
~Chad