Personal experience from a Leaf at -30C the winter before last :D That had accumulated enough heat in it from the previous days ride that it never got to 0C.

That specific car did not have liquid cooling but it did have electric heating yeah.

Sources are many, google it.

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But yeah, if you leave the car unplugged for an extended period in winter (like weeks) then it will drain from maintaining the safe temperature and then there might be damage. Which is why for taking care of your car you leave it plugged in for such situations.