I figured out a few problems:
-DetailCustomizations.cpp actually DOES exist, contrary to what i posted in my question. Looking at it in combination with your answers is making this all much clearer.
-I wasn’t aware of how Modules worked, but after making my own(which was useful since this editor is specific only to my game), registering things in startup and shutdown for the modules makes sense, as well as the RegisterCustomization()s you were referring to.
Anyway, I made my own module, and then set up the correct registrations with it, and now it draws a thing in the detail panel! Thank you for your help.