iPhone 7 died and overheated after dying

Never had battery or overheating problems before. I had used my iPhone without issues 10 minutes earlier at home. 5 minutes later I heard the phone vibrate from a notification. 5 minutes later when I tried to check the phone it was completely unresponsive and very overheated. It would't turn on and kept overheating on its own, so I put it in the fridge. After some hours I removed it and let it get to room temp. Charged it for hours but doesn't turn on. I also tried pressing the Sleep/Wake button + Volume Down button for 10 seconds while connecting to iTunes.

Do you know what problem it may be? I will send it to a small repair shop (not sure if trustworthy) in a small town. I would try to diagnose on my own if possible. I have no access to Apple stores here. I need information to decide whether I should fix it or buy a new one. At least I'd like to have an idea of what could be wrong. I would also like to do a backup.