A good heat pump water heater will have an insulated storage tank and backup resistance heat. Ours has plenty of water for a long shower but if you drain it faster than it can pull heat out of the ambient environment, the resistive heat kicks in (you can disable this if wanted) and you’ll continuously get warm water. We generally only notice that with guests in the winter - in the summer, it’s basically free since we’d be using the main heat pump to cool the house anyway.