I was fortunate enough to have my Prime prior to winter last year, so I was able to capture a few drives as the temperature dropped to share with those curious this year. The electric heating in Prime is provided by a vapor-injected heat-pump. That's the most efficient means of cabin warming currently available for electric-only driving. It operates in temperatures down to 15 F degrees. In conditions colder, the engine joins in to help provide warmth. So, for most of the winter even here in Minnesota, I get to enjoy EV mode. Of course, when the engine does start, it eventually shuts back off to circulate hot coolant as you drive with only electricity. (Watch that data shown in the second and third videos, indicated by the 2 aftermarket gauges displayed on the right.) Here's a video of that electric-only operation at 16 F degrees: This is that same drive at a lower temperature, down to 7 F degrees: And here's one that even here in Minnesota, we fortunately don't have to deal with often:
I've used the heat-pump without the ICE running down to 10°F, though 12°F is the more common point at which the ICE starts.
I think he uses an app called "Hybrid Assistant". Hybrid Assistant - Apps on Google Play Obviously, you need an OBD Bluetooth module to make it work.
The following has a sample of Hybrid Assistant output: In the first post, that data was recorded from the "Torque" phone app, which only requires a $10 aftermarket ODB-II reader.