I'm not sure if you mean hybrid or plug-in-hybrid. That should have been specified.
I have a 2016 Toyota RAV4 hybrid (standard hybrid, plug-in wasn't offered back then). My car powers everything including the power steering and windshield heater from the 12 volt battery, with these exceptions:
- Traction for motors is provided from the high-voltage battery, and as a result of this because the car does not have a traditional starter motor, the engine is started from the high-voltage battery (so if your high-voltage battery doesn't have any charge, charging the 12-volt battery won't make the car start)
- The air conditioning compressor is operated from the high-voltage battery
- The DC-DC converter providing 12 volts is operated from the high-voltage battery, so your 12 volt battery charge is kept topped up by the high-voltage battery
- The car lacks alternator, so the motor-generators are used to charge the high-voltage battery. The 12-volt battery is charged by the motor-generators and the DC-DC converter so there is no direct 12 volt charger.
Usually the idea is that 12 volts is a safe voltage but the high-voltage battery is operating at an unsafe voltage. Thus if something can be powered by the 12 volt battery, it will be powered by it. There are cars with 12 volt power steering, so 12 volt power steering is used. There are cars with 12 volt heated windshield, so 12 volt heated windshield is used.
However, the traction motors require so much power that good luck powering them with 12 volts. Similarly, the air conditioning compressor is quite powerful and the 12 volt battery is not beefy enough for it.