Once you get reference down, electrical ( minus a large amount of electronics ) is a matter of pressure, volume and resistance.
You car runs on 13.8 Volts. That is the pressure. You alternator puts out 14 Volt or more of pressure OF 65 to 150 Amps. Amps are the volume.
The resistance to the flow of the Amps is called...you guessed it Resistance.
Okay, relative to ground, all of your readings with your voltmeter are made with the black lead connected to ground ( your reference ). On the battery not running, 12 to 13 volts. Running will be from 12 to 15 Volts ( the alternator is trying to push more Amps back into the battery ).
If you read the voltage at say the fuel pump, it may be lower because the long wire getting to the pump has some resistance in it.
These are general concepts that may be helpfull.
