Most heat pumps these days are mini-splits, not ducted. And they have a separate unit in each room. Either with multiple compressors outside, or a single one that can ramp up/down.

Heat pumps are useless in hard frost.

The one I own stops being useful at 3 degrees of frost, struggles before then

As I wrote above, the only time I've seen mine falter was at 6F (last year's mega freeze in Austin). It occasionally gets to 13F-15F in the winter, and the heat pump works in this environment without having to resort to "aux heat". I'm sure it's become rather inefficient, but it does work.