Typically the backup is resistive, in a ducted system, it’s basically a big heater coil in the heat exchanger area. One of my two properties has one. It has never been used though, AFAIK, because it hasn’t gotten cold enough yet (and frankly, in Eastern Massachusetts, it is not very common anyway).
If you have propane/nat gas for hot water, you can use that as the backup heat -- just need a combi-boiler with a single zone through a hydronic loop in the air handler stack.
I do not, no gas line to the house. I had an oil furnace which I replaced hydronic with heat pump + resistive backup. Saving about $300/month this winter
