iPhone 15 Pro heating up with protective case

It isn’t a problem, because the phone monitors its temperature and shuts down if it exceeds the safe temperature range. But your iPhone or ANY electronic device will get warm with use; the more energy consumption in progress, the warmer it will get, and if you have an insulating case on it not all of the heat can escape.

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. In particular, fast charging with a 20 watt power source will cause any smartphone to get very warm, so warm it may have to pause charging to cool down after 30 minutes or so.

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, the other 70% becomes heat. And signal strength matters; a 1 bar signal requires that the phone boost its transmitter power to maintain a connection, and it isn’t linear. 1 bar uses 10 times as much energy as 4 bars. You didn’t mention your phone model, but an iPhone 12 or newer (including SE 3rd edition) with a 5G signal uses about twice as much energy as an LTE signal, and 5G towers are still not as densely distributed as LTE, so the signal is likely to be weaker, compounding the problem.

Plastic, silicon, nylon and virtually any other material are insulators. In other words, they don’t let the heat escape and actually retain heat making it warmer. The thicker the case, the more it insulates.

iPhones cool off by transferring heat from the processor, memory, battery, charging circuitry and every other electronic source to the exterior of you iPhone. The better performing the iPhone the warmer it’ll get. The trade off is literally performance vs. temperature.

If you don't mind, I'm going to save this to share with others with similar questions. I've used something similar I wrote on my own, but this one is much better. As always, I give you a tag line as the original author.

You’re more than welcome to use it!