My iPhone 12 keeps freezing and then turns off and eventually comes on again

What does the Battery Health / Maximum Capacity look like?

Settings > Battery > Battery Health > Maximum Capacity.

If Capacity is 80% or less, the battery needs to be replaced.

Perform a Forced Restart on the phone. Follow the instructions very carefully.

Force restart iPhone - Apple Support

There are 2 possible results with the Forced Restart:

1) The phone ruturns to normal operation

2) There is a hardware problem with the phone and it will need attention at the Apple Store or authorized Apple Service shop

Good luck!