Is there any kind of authoritative guide to Apple's Volume Purchase Program, Device Enrollment Program, and Mobile Device Management?

I'm fairly new to the whole world of Apple and macOS. As I'm currently writing my thesis about the possible enrollment of macOS devices via [Device Enrollment Program](https://www.apple.com/business/docs/site/DEP_Guide.pdf) (DEP) (in connecting with some other stuff), I searched the Web, books and the Apple's Developer documentation, but I have not found a reliable guide on how to use [Volume Purchase Program](https://volume.itunes.apple.com/?l=en) (VPP) and DEP (or at least a "quotable" description), only the handbook for ABM (which is pretty good, but does not answer all my questions). Am I just blind or are there some "hidden" gems that i seem to overlook? Maybe there is a guide in [Apple Business Manager](https://business.apple.com) (ABM) directly, to which i don't have access - yet - ?