Hi,
To step down from 20V to 12V:
There are ready to buy DCDC converters. In your case they are called "buck converter", or "step down converters".
Input: 20V battery. Mind the voltage may vary a lot: maybe 15V ... 22V. It depends on the battery type.
Output: 12V. To drive everything but the motor. You need to determine how much current these devices need maximum. Add some margin.
No need to be isolated.
Often made mistakes:
* mind to integrate "under discharge protection". Else leaving the car ON over night may kill your battery.
* wrong wiring of high currents and switched (motor) currents may cause a lot of trouble. From malfunction to EMI problems (in worst case it jamms emergency radio. Not very likely, though.)
* also be sure to use correct wire gauge, you don't want to be your child to be involved in a plastics car fire.
Klaus