I have had my 19 rav4 hybrid heat on and it works great In Kansas, been in 20-30s here. No complaints.Proper answer will be - both.
Both ICE and hybrid battery produce heat, but not through "resistance wires", simply due to normal battery operation.
Presuming that your question is - what provides heat for the interior, answer is - ICE only. This is why hybrids have always had reputation of poor heat producers and ICE runs much more often cold times, as ECM has to somehow have source of heat.
Now, on olden Camry Hybrids there was a PTC heater added to the heater core. THAT actually was resistance based device and worked tremendously well, I had heat in less than a mile, full blast. But, Toyota wisely chose not to install it on Highlanders and, apparently, on newer and new sedans.
So, winter lurking around the corner, you will start seeing influx of rants about hybrids not being as warm as their conventional counterparts. Or, rants about mpg dropping down. Duhh...
You can only go a few miles in EV mode so what difference does it make? The ICE will kick on and provide radiator heat, assuming your radiator is full. About 40 years ago I had a car that had a bit of a radiator leak. When I came stop sign, the heat instantly went away since there wasn't enough water flowing to provide heat through the heat exchanger.Out of curiosity, in EVs do they use heat pump technology to heat & cool the car? If so I would have expected hybrids to use the same technology for cooling and for heating until the motor starts producing waste heat.