Why my iPhone is overheating?

Keeping iPhone, iPad, and iPod touch within acceptable operating temperatures

https://support.apple.com/en-ie/HT201678

My iPhone has been overheating and being in the sun or an overheated room is NOT the problem. There is some kind of a system or software issue that needs to be resolved. I found a non-Apple post that said to turn off background app refresh - phone seems better since doing that, but should that be necessary?

Do you see on the screen a temperature warning and message that your iPhone needs to cool down before you can use it again? If not, then your device isn't overhearing but it is getting warm or too warm during the use.

So? What is it happening precisely? The better you try to describe what exactly is happening, the better response you might get.

Otherwise, either contact AppleCare where they remotely can run diagnostics on your device or bring your iPhone to nearest Apple Retail Store or Authorised Service Provider for hardware test.