First things first I would decontaminate the logic board. Get some 91% isopropyl alcohol and put the logic board in it for a few minute and lightly scrub with a toothbrush. Clean all of the connectors and everything like that. Then clean the rest of the phone if desired. Try replacing the back camera and the flashlight. Hopefully this helped
It might be a bad power supply or the cable that plugs into the outlet. Try the cable then the power supply. If neither of those work it’s either a motherboard issue or an issue with the outlet that the cable is plugged into.
I think this would be caused by a torn cable for something in the phone. Try just getting the board, screen, battery and charging port and see if it boot loops then. If it doesn’t problem solved. If it still boot loops go through the process of elimination. Replace the charging port, battery, screen and it non of that works it’s a board level issue.