iPhone 12 Pro Max temperature

Depends on how hot. Also, we’re you using the camera for an extended amount of time? Recording a video or anything I mean. If you’re extremely concerned though have it inspected at an Apple store if one is available to you or an authorized Apple repair facility so they can determine if there is an issue since it’s covered under warranty

Normal of all batteries to get warm during charging or heavy use.

It will turn off if it gets too hot.

Yes, your phone will warn you if the temperature becomes too warm for normal operation. The apps you used would cause the battery to work harder, thus becoming warm.