My suggestion to you is this. I wouldn't worry about how every single iPhone varies in temperature. It's depending how you're using your iPhone and what for that sometimes it will warm up considerably. But that doesn't make the iPhone necessarily defective or indicate something is wrong.
While the high temperature might be normal, it isn't good for the battery.
https://support.apple.com/en-ca/HT201678Using an iOS device in very hot conditions can permanently shorten battery life.