It sounds like this is a common problem with the iPhone 7 256GB, but you have a plus. The plus is mentioned [https://forums.macrumors.com/threads/iphone-7-randomly-overheats.2000146/|here], and it seems to be a software issue:
"Iphone 7+ - I went in and killed the background refresh apps and rebooted my phone a few times. Problem solved and is back to normal. I have experienced this on multiple iPhones so not the least bit worried." -lykbutta
[https://discussions.apple.com/thread/7671057|Here]'s another place you might look for answers.
However, none of the other comments I've seen have said specifically ''where'' it's getting warm. That made me curious. According to the teardown, it's likely the bottom of the battery and/or the power amplifier modules directly above the home button (as seen in Step 14, light and dark blue boxes). As for the camera part, it's either the camera itself or the screen. If it's a little below the camera, I'd think it's the wifi/bluetooth chip (Step 15, orange box), but I'm honestly just speculating. It could be the camera or the screen itself, but I'd expect it to get warm fairly evenly if that were the case. Again, I don't know for sure.
I hope some of this is helpful. Good luck!