Best Practices for Customizing or Modifying a GitLab installation?

Newb to the boards, sorry if wrong section. (SystemAdministration? HowTo?)

I’m doing modifications of a local gitlab installation. I will not be the maintainer, who may have to keep this thing going long after I’m gone so I want to make sure that I follow a path that will let the maintainer keep gitlab updated even with my customizations.

I’m unsure how the current installation was done, besides “it wasn’t omnibus.” I’m awaiting those details.

What documents should I be reading? Is there a “patches” or “hooks” that can be re-applied even when gitlab itself updates?