In an EV, the 12V battery typically powers the computers, and all of the regular stuff powered by 12V in an ICE car, door locks, blower fans, lights, etc.
The difference is that there is a DC to DC power converter which provides 12V to charge and maintain the 12V battery and 12V battery bus during operation. The converter provides power from the high voltage traction battery (hundreds of volts) to the 12V battery and battery bus.
There are different models on how the 12V battery is charged, sometimes only during operation, others maintain the battery when charging, or even when dormant (from the traction battery).
The 12V battery can be of small capacity, because when driving, most of the energy is actually being supplied by the DC to DC converter (Chevy, for example, calls it the auxiliary power module (APM)). For technical types, you can think of the 12V battery during EV operation as a capacitor on the 12V bus.
Another tip, which may or may not apply to VW EVs is that beginning with Volt may EV drivers also carried a 10 mm socket wrench, because some odd/weird computer glitches could be cleared by momentarily disconnecting the 12V battery (tens of seconds to minutes) to "cold boot" the car. There may be cases where this fix is not good (e.g. if it resets fault codes, or loses user configurations). OTOH, if you are stuck in the middle of nowhere, and it fixes the glitch ...