Probably a combination of what surface you have it sitting on and which case you use (if any).

And ambient temperature and air movement. Anything that impacts heat dissipation.

I was in Phoenix AZ last week when it was 115F and my phone’s hotspot shut down because the phone got too hot.

You were outside? I mean...

Not exactly outside, but in a building without air conditioning yes.

Yes it was awful.

Probably because you're close to whatever tower it's connecting to and the radio bits are not having to spend a bunch of energy screaming at the top of their lungs to be heard by the tower.

Even plastic clad android phones will get hot if you try and make a long phone call from BFE.

also the Mac may detect it as a metered mobile connection and defer/throttle background usage (updates, etc).