For some number of cars there is a bit of air leakage out through the mirrors. Keeps them clear of new ice and will clear them over time frozen or not.
The other case is all cars have an air-flow directed at the windows between driver and mirror. If it is an integrated mirror (not out on a pole) this does help create a warm air pocket just outside, just enough to help keep that clear.
Note: never stated as a promise by automakers, but they do design for that. It may not even be conscious. They may have arrived at this though all weather testing of their product. Mirrors with these features just stay clear (more often).