Your C-h f is not the standard C-h f (M-x describe-function), describe-function doesn't provide "References ..." information at all. To find the source of apply, use C-h f apply, it will say
apply is a built-in function in `src/eval.c'.
src/eval.c will be clickable, then click it to go to the source. If you didn't install Emacs from the source code by ...
You need to distinguish between
interactive command. The latter is also a function, but with the addition of (interactive) to its body.
You can run the latter, but not the former, by binding commands to
keys or calling them by name via M-x. See the elisp manual
links above more more details.