i have a Series 8 Watch.
i have an iPhone SE 2.
i have my iPhone registered as a bluetooth device to my car's audio/mobile system.
i have my Watch setting so that when i long press the Crown it lets me use Siri.
when i long press the crown on the watch and ask it to call someone, it will make the call for me.
if im in the car, the call is put through the car audio system.
the way it does this is its actually using the iPhone to contact the car's audio system.
the watch is not registered as a Bluetooth device to my car audio system.
now, some other info.
my Watch is a cellular model. so it can make calls even if the iPhone is powered off or not connected to the net in any way.
when im not in the car, and just use the watch to make a call, as long as my iPhone has power and is connected to the net, the watch initiated call will actually still "make" the call using the iPhone (there could be a proximity / bluetooth factor here also im not sure).
but if your iPhone is off and not connected to the net, then i doubt putting the call through the car audio will work.
finally, of course if my iPhone is powered off or not net reachable, the call initiated on the Watch is placed directly from the Watch itself, using its own internal circuitry. and since its not a registered device with my car audio system i dont think it will work through the car audio system without going through the iPhone.
for me, while driving, its a lot easier to make calls using just a simple one-action of a long push on the watch crown and asking Siri to dial someone.
the iPhone is doing all the heavy lifting in the middle, with just the call initiation being done by the Watch, and the audio output being put through the car's audio system.