A very common issue with the iPhone X series and 11, is a tiny bit of water landing on the proximity sensor. This usually causes the phone to reboot all the time.
The proximity sensor is on the flex cable that has the earspeaker attached to it.
Simply disconect it, and leave it disconected for now. Boot the phone and see if the phone still reboots after 3 min. If it does not, then that is probably the issue. Replace the proximity flex cable.
Note: the proxi is part of the faceid circuit and you will loose that functionality, if you replace that part. You can transfer or repair the original proxi if you can micro solder.
If disconecting the proxi did not fix it, disconect everything besides the screen, battery, and power button. Boot the phone and test for bootloop.
If the bootloop is gone, start recconecting parts one by one while testing for bootloop. You will find the part at fault.
If it still bootloops disconect the power button and connect the port instead, and prompt to boot via the port and test again.
If the problem is still there, you have narrowed it down to the motherboard.
I hope this helps