From the POV of Apple it is stolen, and it could be stolen. Contact the guy who gave it to you.

You don’t, Apple Business Manager will pickup the serial and register it for MDM by applying whatever profile.

The only way is either with proof of purchase or by contacting the MDM admin.

In my looking at stuff like ipads, apple will only help you if you have proof of original purchase. Being a 2018 that’s probably long gone even if they didn’t steal it.