How are a Tesla's batteries charged below 0degC
The battery pack includes a whole thermal management system to keep them within optimal temp range.
More here: https://www.easyelectriccars.com/why-do-tesla-batteries-not-overheat-teslas-battery-cooling-system/
And here, with some detail. https://teslamotorsclub.com/tmc/threads/tesla-thermal-management-system-explanation.88055/ tl; dr: there is a 6kW heater to bring the batteries up to temperature when needed.