If you've ever been in an area where your phone is
just barely connected to a tower, and is struggling to keep a connection (and has no better tower to switch to), you'll notice your phone will get
quite hot—hotter than it would normally let itself get before thermal-throttling. (Because it's not the CPU getting that hot; it's the antenna+baseband.)
Often, this heat will be localized to where the antenna is located—I've noticed that my iPhone 8 will sometimes feel burning hot along its right edge, for example, and no amount of closing applications or disabling radios will cool it down. The only thing for it, usually, is to turn it off entirely, such that the baseband stops receiving power. (For some reason, upon turning it back on, it doesn't heat back up, even if I haven't moved—perhaps because the baseband's criteria for connecting to towers is stricter than its criteria for staying connected to towers.)