iPhones are designed to operate from 32 degrees to 95 degrees Fahrenheit, which is equivalent to 0 degrees to 35 degrees Celsius. You'll find that this is the case with most electronic equipment, from televisions to gaming consoles. Once the optimum temperature threshold is exceeded and a warning screen appears, the phone will automatically enter cooling mode.
iPhone overheating iOS temperature warning

Here's what happens in Cooling Mode:

    The screen loses brightness or turns completely black
    Charging becomes unresponsive or very slow
    Camera flash is disabled
    weak signal

However, you will still be able to make emergency calls.
What can cause a high temperature?

There are three main causes of device overheating.
Processor under heavy workload

If you are playing graphics intensive games or editing and exporting 4K video, you are likely to see rapid battery drain and overheating. This is due to the fact that there are no fans to cool the processor, as in laptops and desktop computers, and the aluminum casing is not able to expel sufficient heat on its own. In most cases, the device will simply overheat. This should not be enough to cause damage or even trigger a temperature warning screen.
You left your device in a hot environment بيئة

Keeping your iPhone in a hot car or in direct sunlight is definitely something to avoid. This is greatly exacerbated if your phone has a black case. Black absorbs all wavelengths of light, unlike white, which reflects most of the light. This alone will cause it to overheat, which can damage the battery and other components.

So, if you're spending a day at the beach, keep your iPhone under a light-colored towel or in the shade. Parking the car for a while? Keep your phone hidden in the shade and break windows - or take it with you. I know it sounds relatively simple, but these two reasons alone - both because the device was left in a warm place - are the most common causes of an overheating device.
Battery malfunction

A failed battery can expel a lot of heat. If you think this is the case, immediately turn off the device and service it. This is rarely a problem, since iOS has steps to follow in the event of a battery failure - essentially, iOS prompts the device to shutdown without user input. You can run a battery diagnostics to check for signs of battery failure, how to avoid it, and steps to service your iPhone.
Additional steps

Definitely don't put your device in the fridge or freezer hoping to speed up the cooling process. While this is a way to fix the problem, it will do more damage than it might help. Due to the rapid increase and decrease in temperature, the total battery charge capacity will be reduced due to damage.

However, there are a few things you can try instead include:

    Remove the casing so the heat can escape more easily
    Close all running apps and disable background app refresh
    Entering low power mode. (This will turn off multiple features, but you can turn them off individually if you depend on them.)
    screen dimming
    Turn off location services, especially energy-intensive step-by-step map directions
    Turn off bluetooth
    Keep the device away from a warm environment
    Run battery diagnostics
    service your device