What you describe sounds like normal performance. The back temperature around the camera plateau will get warm when the cameras are used. Certain use such as 4K 120fps is very CPU and GPU intensive but all recording modes produce a lot of heat. Aluminum is a much better conductor of heat than glass and the heat from the internal components is conducted to the other shell. Cases act as insulators and will keep the heat trapped and contribute to the rising temperatures.
Apple builds in to the systems multiple failsafes that will throttle down iphone components and thereby produce less heat. If your iphone is approaching a true danger zone, it will temporarily stop and display a high temperature warning. This warning stops the iPhone before an internal damage is done.
You are welcome to have Apple perform diagnostics on your iPhone. It can be done in person at an Apple Retail Store or at an Apple Authorized Service Provider. You can also contact Apple Support and request remote diagnostics.
If you’re in the US, please call 1-800-MY-APPLE, or outside the US, consult the support articles below. In Canada, call 1-800-263-3394.
Contact Apple for support and service - Apple Support
Genius Bar Reservation and Apple Support Options - Apple
Explore Service Locations