Just wondering, can a heat pump work at very low temp ? Like -30c.
No, the heat pump will not be able to scavenge heat from the ambient outside air at this temperature. Instead, the car will fall back to electric heating in these conditions (perhaps an oversimplified description). The car will heat the evaporator loop with waste motor heat, similar to how Model 3 currently does battery heat, which will then be transferred to the cabin via the heat pump.
The heat pump will really shine in cool conditions, particularly above freezing. The colder it gets, the less efficient it will become, until it reaches ~parity with electric resistance heat.