Any device that uses Lithium technology batteries (which means every cell phone, tablet and smart phone on the market) works within the same laws of physics. Lithium batteries don't work below freezing. Apple can't change that. That's why cars that have Lithium batteries (Teslas, newer Priuses) have battery heaters. which is also why a heated case, which would work with any phone, is the best approach.
All of my winter coats have pockets for phones. The opening is on the surface of the coat (usually the lapel), but the rest of the pocket is inside the down lining, so body heat keeps the phone warm enough. Taking it out for a few minutes to use it works fine, but it doesn't solve the problem of having it outside and in use for hours. But then again, the phone's battery won't last for hours between charges, also.
Another thought just occurred to me. Get a hefty battery booster like the Mophie or several others. One that is not a phone case, but instead connects with a cable. I've got 3 right now for different situations. Keep the battery pack inside your clothes, connect the phone with the cable to keep the phone charging, as the phone gets quite warm when charging. While charging the phone would probably stay warm enough to keep operating under a larger range of temperatures.