Heating issue while charging iPhone 12

It’s called the Second Law of Thermodynamics. Anything that creates or uses energy does so with some loss. That lost energy is expressed as heat. So when you charge the phone it generates heat in the power source, heat in the charger circuit in the phone, and heat as the energy goes into the battery. The faster the charging, the more heat is generated. Likewise when you discharge the battery; not all of the energy from the battery gets to the circuits that use it. Some of it becomes heat. If you use the cellular network for voice or data, converting energy to radio frequency signals is very wasteful; only about 30% of the energy that goes into the network components comes out as radio signals.

As a lighter view of the laws of thermodynamics, the Battelle Memorial Institute many years ago published a lay-persons explanation of the 3 laws of thermodynamics:

1. You can’t win (Energy cannot be created; that’s why there is no perpetual motion)
2. You can’t break even (There is some energy loss in every process)
3. You can’t quit the game (It is not possible to reach a state of no energy - absolute zero)

More details? Are you using a case? Is it uncomfortable to the touch? Is it going into thermal shutdown with a screen message that it can't be accessed until it cools down?