Iphone 12 heating issue

Hi there! Sorry to hear about your overheating phone :( Here is an article that explains overheating or cold phones and what you can do to fix the issue

If your iPhone, iPad, or iPod touch gets too hot or too cold - Apple Support

Sometimes if you swipe up from the bottom of the iPhone, you can close running apps. This helps with battery life and keeps the phone running per spec

The latest iOS update for an iPhone 12 would be iOS 15.6

Did you already try to set your phone back to factory settings, using your computer?

Restore your iPhone, iPad, or iPod to factory settings - Apple Support

This will also install the highest possible iOS version and you will be sure, that the software has been installed correctly.