The heater work with temp sensors.
When the temps are going to -20 celcius the BMS tell the VCM to activate the pack relay then the relay for heating have power on his contact to supply the heating pads.
VCM and the BMS work together on this.
I replaced my 2011 battery pack with a 2014 one who have the heater and I studied how it work.
Best is to buy heat pads for oil pan for gaz car.
You stick four pads of 100Watts or 6 of 50 watts and there is a 120vac automatic switch for this.
Just plug all together and plug in a 120 vac.
I remember one man done this and it work very well