unable to update/restore iPhone 12

If you are waiting and it’s still not start try last time with complete reset (data will be lost) so, if it’s not start maybe you have to take an appointment from an Apple store, it's time to replace the rear system (back cover)

iPhone Repair - Official Apple Support

dutchliar wrote:

I have a brand-new iPhone 12. The initial start-up worked great, and it then asked to update to iOs 15.5. During the update it got stuck on the apple logo screen. I tried a hard reset (vol up, vol down, hold power button), same result. I then connected to my Mac Mini and entered the restore page. It shows up in Finder and I selected to upgrade. It took a few resets (time out during downloading), but I was finally able to get it through the full upgrade process. It says to wait until the iPhone restarts and then it should appear again in Finder. However, it restarts back into the apple logo screen, and is stuck again.....

What else can I do?

Wait. It may take several minutes to restart after updating.