I recently spilled water on my 15 pro which broke the screen, however vibrations and sound was still working. I went to a repair shop where we tested if a new screen would work. We tried it without the screen being attached to the rest of the phone and everything worked as usual (touch, Face ID, cameras, etc.) He told us to give him 10 minutes to apply the screen and that he would call us once all was done. After 15 minutes and no call we return to the shop, with him telling us that the phone went into some sort of loop and was constantly restarting. he said our only option is to try to update it, which we did... which didn't work. He told us it is likely that the motherboard is damaged due to the water and that there is basically no hope for it to be repaired. We applied the broken screen again and went home without success.
Also. when we got home, the phone no longer powers on and vibrates like it did before the repair, so we are kinda suspicious of the repair shop thinking they maybe broke something when they tried to apply the new screen. (It was laying for 2 days being able to vibrate and make noise before we went)
My question is however if there is anything else that could cause this restart loop, because I would much rather pay for repair rather then buying a completely new phone.
NOTE: An official apple store repair is off the table since we have a "non-apple" battery inside.
TL;DR: Phone is stuck in restart loop after water damage, repairman says it is faulty motherboard. I'm wondering if it could be something else (It isn't a software update nor a storage issue)