iPhone gets hot whenever I’m using Instagram

If you are using the camera it requires a lot of energy, and thus the phone will get warm and use a lot of battery power when on. If you are using it with a cellular connection rather than Wi-Fi cellular data (especially 5G) also uses a lot of energy.

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. 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, 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.