The iPhone gets hotter

Hello,

Your device might get warm

When you use the device or charge the battery, the device might get warm. You might notice that your device feels warmer in these situations:

• When you set up your device the first time
• When you restore from a backup
• When apps reindex or reanalyze data, like Photos tagging for faces, places, or keywords after a software update

These conditions are normal, and your device will return to a regular temperature when complete.

Here are some of the higher ambient-temperature conditions and activities that might cause the device to change performance and behavior:

• Leaving the device in a car on a hot day.
• Leaving the device in direct sunlight for an extended period of time.
• Using certain features in hot conditions or direct sunlight for an extended period of time, such as GPS tracking or navigation in a car, or playing a graphics-intensive game.

More info here:

Keeping iPhone, iPad, and iPod touch within acceptable operating temperatures - Apple Support