Air-to-water heatpumps are the norm here in the Netherlands. We just put a unit outside, with either refrigerant lines or simply the heated water lines running from the outside unit to an inside unit.

Probably helps that we are in a pretty moderate climate here. And thanks to global warming it's been a while since it got really far below zero degrees celsius.

When the outside unit really can't keep up with demand, it may switch to a gas heater (hybrid) or an electrical heating element (all-electric)