Prius sold in some markets have supplementary electrical resistance heaters, but after all these years since 2001 S. Calif. purchase, I still don't know if mine has them. It has kept me plenty warm when the outside temp. was -19F. There is a electrical motor to circulate the hot coolant to the heater core.

Before the gas engine cools off enough to short-change you on heat, it will restart for one of the other two reasons (low HV battery voltage or low intake manifold temperature).

DAS

edit: And Schwa replies first, by a nose!