Is there any kind of authoritative guide to Apple's Volume Purchase Program, Device Enrollment Program, and Mobile Device Management?
I'm sure if you'd searched the Web just with the keywords that you have listed, you'd get a lot of official resources right on the first page of the search results.

But nonetheless, here are some links to get you started:


Apple Deployment Programs - Volume Purchase Program Guide
Apple Deployment Programs - Device Enrollment Program Guide
Apple Deployment Reference - Mac
Apple Deployment Reference - iPhone and iPad
Mobile Device Management Settings
Apple Deployment Programs
Upgrade from Apple Deployment Programs
Volume Purchase Program
Apple Business Manager User Guide
Business Support
Getting Started Guide - Apple Business Manager
Upgrade your organization to Apple Business Manager