So how do you control DC?
Simple relay logic. The main battery supplies voltage to a few flight deck control switches. One of them is the 'Battery' switch. Closing this provides a signal to a DC contactor (big relay) to connect the battery to the battery bus. Many of the systems control computers (including electrical bus control logic) are powered from this bus.
One can also plug external power into the airplane. This powers a utility bus and a few other systems used for ground operations like lights, cargo door controls, etc. In this case, the main battery is not involved.
This is a general, high level explaination. The specifics depend on exactly which model you are interested in.