iPhone stuck in restart boot

If the phone restarts a few seconds after shutting off I doubt it would be a battery issue since the battery would only be out of action for a few seconds, which if the battery was the problem I would expect it to take a lot longer to start booting again.

Try unplugging the ear speaker cable completely and test the phone. Face ID will not work with it unplugged however it will narrow it down whether or not the ear speaker assembly is the component causing the restart.

If the phone still restarts with the ear speaker unplugged, try unplugging flex cables one by one until the issue stops, which then it can be confirmed which flex cable may be causing the issue (all flex cables besides the ones which are important for the phone to boot, like screen, power button and battery). Make sure to turn off the phone and unplug the battery before unplugging anything.

However, even after unplugging everything and the issue still persist, it may be a motherboard issue. Try seeing if you can feel the motherboard getting really hot somewhere around the time it would restart, which if you can feel it getting hot there may be a short. From there the phone will need soldering work.

Hope this helps!