The frost is forming on the inside because there’s moisture in the car. Perhaps it came from snow on your shoes when you get in. Perhaps there’s a leak somewhere. For whatever reason, there’s enough moisture in the car to condense on the inside of the windows and freeze there.
Do you keep the climate control setting on “recirculate?” If so this might be contributing to the problem. Set the control to “outside air” and operate the AC to dehumidify the air. Yes, it’s OK to run the AC in winter. The heater will still work, even with the AC on.
I suggest checking around on the floor to see if there is wet carpet anywhere. Lift the floor mats and check under them. Leaving a window open slightly on a warm, dry day might help get rid of the moisture, assuming there isn’t a lot of it.
If there’s water somewhere, you have to get it dried out in order to stop the condensation.
Getting rid of frost on the inside is easy; turn on the defroster and set the temperature and fan control to high. The frost will melt within a few minutes.