Check/find/locate the customized function/command of a specific project in Emacs conveniently/quickly/efficiently.

[Hongyi Zhao <hongyi.zhao@gmail.com>]

I'm currently learning the configuration used by doom Emacs [1]. For
example, the following code snippet shows on
<https://github.com/hlissner/doom-emacs/blob/2731685095d1e6101b3215aa689426e1834ce00f/modules/lang/latex/config.el#L256>:

(use-package! company-math
:when (featurep! :completion company)
:defer t
:init
(add-to-list '+latex--company-backends
#'+latex-symbols-company-backend nil #'eq))

In the above code snippet, there are some customized
function/command/variable, i.e., use-package!, featurep!,
+latex--company-backends, and +latex-symbols-company-backend, which
are not so easy to understand just by their names. So, I want to
check/find/locate the corresponding definitions for them in Emacs
conveniently/quickly/efficiently. Because they are customized things,
the
`M-.' doesn't work for them.

Any hints for this question will be highly appreciated?

[1] https://github.com/hlissner/doom-emacs

Regards
-- 
Assoc. Prof. Hongyi Zhao <hongyi.zhao@gmail.com>
Theory and Simulation of Materials
Hebei Vocational University of Technology and Engineering
NO. 552 North Gangtie Road, Xingtai, China