After some time, I realised that my iPhone X is getting very hot on the back around the camera while browsing Safari, not even playing a game or youtube. I'm running the latest iOS 12.3.1. Is it happening to you also?
Do you have any Website open in your tabs that may be loading/playing videos? Best thing to do is have an Ad Blocker like Crystal, it helps a lot in preventing unwanted ads from trying to load or reload while browsing multiple tabs.
Also another feature that I found to eat battery and cause an iPhone to heat up is in the FaceID & Attention settings under Accessibility, it’s this option circled below, at random times it would cause my XS Max to heat up a bit and drain battery, seems it is still a feature that requires refinement: