iPhone often freezes nowadays
Solution 1:
The term "freezing" here should not be interpreted literally. Freezing simply means that the display stops updating, audio stops playing or similar (i.e. nothing is happening) - it doesn't have anything to do with hot or cold environments.
The behavior you're seeing sounds like a hardware defect.
You mentioned already having tried upgrading iOS. I would take a backup of the phone, and then do a complete restore of the iOS software - erasing all contents and settings on the phone. If it still freezes afterwards, it is most probably a hardware problem.