I live in MN, and if it’s cold out, I keep my phone next to my body somehow, like a pants pocket or inner jacket pocket in the chest area. This keeps the phone warm enough to work for a while at any temperature you would attempt to use a phone in (I don’t try to use my phone outside when it’s -20F because I don’t want to remove gloves, but 0F is fine) and if it’s too cold, you can rapidly warm it up by placing it between your legs/against your skin/etc.

In he summer, I don’t leave my phone in direct sunlight for extended periods or else it will overheat. Simply placing it in the shade or in a pocket prevents this. It usually only hits 100 here, so I don’t have experience with the 100-120F range and overheating.

Tl;dr Prevent your phone from reaching extreme temperatures and it will work fine.

I’ve only had problems even well below 0F in Minnesota using old phones.

It’s just battery chemistry, current capacity and voltage are depressed by low temps.

Some phones aren’t engineered very well and regularly pull current near their battery’s capacity which is just poor design.

Of course 3/4 of the people responding to how cold can a phone get are from Minnesota. Lolol. Edit: I am also

I’ve found this is very much a function of how new/good the battery is. I’ve been out skiing in ~0 degrees for a couple hours with a newer phone and not had trouble and I’ve had the same phone a year or two later go dead after 45min in 20 degree weather. My current iPhone 12 Pro lasted 45 min at -15 this winter and didn’t run out of battery. I keep my phone in an unheated water bottle carrier

My OnePlus One (old, I know) worked just fine this winter. I was able to read on my phone the entire 20 minute walk to the store and back. It's when I got home into the warmth that it would end up rebooting/saying the battery was empty.

It's possible that the phone generated enough heat to stay warmer though. The main problem with operating it was actually how cold my hand that's holding the phone gets since it doesn't move like the other hand does. I had to wear double gloves.

Every iPhone I've had has worked fine outside in Minnesota winters. The phone uses energy to function and so probably is hotter than the surrounding air.

Samsung used to guarantee their smartphones to operate down to -4°F. Not sure they still do. Having trouble finding current information online.