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