@iMedic, the person who made this video likely won't respond. This came from likely an iPhone factory or similar in mainland China. Your communication will probably be banned by the Great Firewall of China. ;-)
a) The first screen is a warning saying this is for internal industry use only. If you have an issue with apple ID, then contact Apple.
b) Still here? Ok, well what it looks like, is the person desoldered the master CPU, baseband, baseband CPU, touch ID button and flash memory. The the person soldered on a new set from a different phone. Then the firmware was reinstalled. It looks like whatever new chips he put on, the firmware reinstallation allowed a new set of Apple security measures to be put in place.
c) If you do follow this, you have some insane soldering skills. The risk is pretty high. There are iCloud removal services on eBay if you care to look around. Or if you have proof of purchase, Apple can remove it for you for free without the surgery.