Why can the phone get very hot from the game?

Question

Unity

I actually made the game 3 in a row and launched it on the phone, the phone warmed up pretty much in an hour of the game, and the charge sat well, as if I was playing some kind of MMORPG, so I’m trying to understand why?
So I’m wondering why it’s so, the game wouldn’t say that it should sit down and warm like that, after all, what is 3 in a row ...
How can you find out what exactly “strains” the phone?
After all, everything works fine, without brakes or glitches, but the heating and the battery ... I didn’t notice this in other similar ones ...