In addition to what the other anons said, I'd recommend to not keep the camera exposed to the cold longer than required as the thermal expansion coefficients of the different materials are different and the electronics also don't like overly low temperatures.
(same goes for phones)
I'd build some sort of thermal protection for the camera and keep the phone and spare batteries for the camera near your body.
I've used my EOS 2000D in -15°C for about 2 hours on a tripod just fine, only certain lenses have issues with the grease getting thicker.
(shouldn't be an issue for the kit lenses though)
With all electronics you will see a decline in battery life, that's normal.