There is lots of good information here, and for sure, it is much simpler than we often make it out to be. Think of the electrical current as if it was water in a garden hose. The battery is the reservoir, or water supply. The electron flow though the wires is like the water flowing through the hose.
One thing that has been overlooked and not mentioned is grounds, and connections. Without good grounds, and clean, tight connections, there will be problems. Make sure all the connectors are clean and shiny, and use dielectric grease to keep them that way. Grounds too.
The electrical system on your car is a 12 volt system. That is nominal voltage. Alternators generally put out around 13 to 14 volts, so when the car is ruing and the alternator and the regulator are functioning correctly a voltmeter will read higher that the nominal 12 volts. And you should see that same voltage anywhere in the system, even at the back of the car where the fuel pump is.
Also not mentioned here is the use of the ohmmeter function of the DVOM. Checking for excessive resistance is a great way to find poor electrical connections. Below is a link to some video that may help you figure this out.
Check out YouTube for how to use a DVOM.