Consider a van or RV that has two 12V DC batteries. The starter battery is used for normal automotive stuff, most notably turning over the starter to start the engine, as well as powering loads such as the radio, headlights, and AC when the engine is off.
The coach battery is used to power loads such as a water pump, the cabin fridge, cabin lights, etc.
Both the starter and coach batteries have their negative terminals connected to the chassis. The loads powered by these two batteries have have their positive wire connected to the positive terminal of their respective battery as you would expect: The starter, radio, headlights, and AC have their positive wire connected to the starter battery; the water pump, cabin fridge, and cabin lights have their postive wire connected to the coach battery; and, all loads have their negative terminal connected to a nearby chassis bolt (which may or may not be shared with another load's negative terminal connection).
Consider the engine is off, and the radio is turned on, and the fridge is running. The radio's positive wire is connected to the starter battery +, and the fridge's positive wire is connected to the coach battery +. No bus bars or fuse boxes, for simplicity's sake.
Both the radio and the fridge have their negative wires attached to the chassis; perhaps very close to eachother, or perhaps even on the same bolt!
Now, I've always understood electrical charge as a flow from one point to another: How do the electrons flowing to power the radio know to return to the starter battery, and the electrons powering the fridge know to return to the coach battery? How don't they get mixed up?
I am asking this in the context of trying to understand how a smart shunt, connected as the first and only item to the negative terminal of the coach battery, and then to a chassis ground, is able to compute the energy usage of only those items connected to the + terminal of the coach battery and not of those items connected to the + terminal of the starter battery, since they share the same ground.