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My iPhone 12 mini is like that- gets noticeably warm on the back with the most basic use, and on random occasions gets downright hot.

Didn’t start doing that until a couple months ago— I’ve been thinking if the 13 series run cooler, it would be worth an upgrade.
What version of iOS are you running? My 12 minis were like that for awhile but the last version of iOS, 14.8, seems better. iOS 15 on my 13 Pro has some quirks.
 
I think I've narrowed it down to Spotify, which has a reported bug that causes it to run hot (currently depleting about 20% of my battery a day, even though most of my listening is downloaded playlists etc.)
 
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Try keep browsing photo app for like 5-10mins, you will notice the phone will be getting warm

no sure if it is a bug in ios15
 
It might be the "Next Door" app I am using. According to the battery info it chewed through 78% of it. Insane. Must be a buggy app (well, I know it's a buggy app), just didn't know it was that bad.
 
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