There’s a carefully designed process that involves sealing the phones in faraday bags so they can’t communicate with the outside worlds, and then replacing/reprogramming certain components so the phone works outside of the Find My ecosystem.

These phones are reprogrammed and re-chipped en masse and then sent all over the world. See all those “mobile phone repair” shops all over your high street/strip mall? they’re all getting their stock from the same place.

Apple operating the hardware pairing scheme is just a cat and mouse game with this industry.