For what it may or may not be worth, the way direct current electricity works is simply this: all the electrons want to go home right now. It is what they exist for, actually. In a system which has negative ground "home" is ground and all the + electrons want to go there. Management of those electrons to make them do just what we want them to is the trick. Some of them may run a clock, some may run a radio, others may illuminate headlights and other lights, some may heat up a cigarette lighter or run a heater fan. They might be directed to a USB port where they'll charge your phone. And it is all about the positive guys (+) wanting to make it to the negative side (-). Alternating current electricity is different, but you don't have any of that in your pickup. If you want a nice book on the basics of electricity, download this:
https://www.zpag.net/Electroniques/Kit/Getting_Started_in_Electronics_-_3ed_-_[Forrest_M.Mims].pdf
