Ground source heat pumps are available that go down significantly lower than 30F. They are more expensive. Though running the electric wires is also very expensive due to their inefficiency. Typical heat pumps are often better suited for warmer climates because of this
It also appears that the tech for the more typical air source heat pumps has improved significantly in recent years which makes it more viable for colder climates
I believe they run a heat conductive fluid through heat exchange coils in the ground. This allows you to pump heat into the ground during winter and extract said heat (not sure how accurate this is) during the winter.
You're not storing the heat that's underground, it's just that the ground is a huge thing with massive heat storage capacity that doesn't notice weather as much as above ground does.
If above ground ranges from 0-100 degrees F, underground likely ranges from like 64-68 degrees F, which makes it really energetically "cheap" to get to your preferred temperature range, heating to idk 70 at most and cooling to probably not even 65.
It also appears that the tech for the more typical air source heat pumps has improved significantly in recent years which makes it more viable for colder climates