My iPhone 12 is frozen and I can’t power off

Did you read this from the related article in the upper left corner? After you force restart the phone using these instructions, see if you can shut down normally.

On your iPhone 8 or later, including iPhone SE (2nd generation)

1. Press and quickly release the volume up button.
2. Press and quickly release the volume down button.
3. Press and hold the side button until you see the Apple logo.
4. 
5. 
   
6. If your phone doesn't turn on, follow these steps to check your hardware and charge your phone. You might need to charge for up to an hour.
7. If your phone still doesn't turn on, contact Apple Support.