When we left our story we had a signal coming from the strings. But from there a lot still has to happen before it reaches your ear.
For instance, most electric guitars have two pickups and many have three. They can be used alone or in various combinations, by way of a pickup switch. There might be multiple switches, or a given switch might have as many as 5 or more distinct positions.
OK, but why? Selecting the pickup closest to the bridge gives a brighter signal that stands out more for lead work. The one closest to the middle of the string gives a "rounder" tone with more emphasis on the fundamental frequencies, and is more commonly used for rhythm work.
That's if you've chosen only one pickup. Often the wiring will permit selection of multiple pickups. The results can vary from a fuller sound to the "out of phase" sound like the lead at the end of this one Prince song with a name I can't remember ("Let's Go Crazy"?). Or each pickup can have its own output jack so its signal can be processed separately.
OK, we've got a pickup signal or two, but that usually doesn't give us enough control of the signal. So usually the guitar itself will have volume and tone controls as a minimum. If there are multiple pickups, there may or may not be multiple volume and tone controls.
The volume control ordinarily will be just a potentiometer, or variable resistor. It can't add anything to the signal, but it can reduce its voltage, which ultimately lowers the volume. It can be manipulated readily in live performance, often to cut off the "attack" of a note - one example of the sound would be Eddie Van Halen in "Ain't Talkin' Bout Love".
The tone control is another pot with a capacitor added. With the right configuration, the combination permits reduction of the strength of the higher frequencies wrt the lower, with the capacitance determining the frequencies that are affected. Manipulating the tone control gives a "wah wah" effect like what is heard in the long lead in Guns 'n' Roses' "Sweet Child o' Mine".
In practice much of the volume and tone manipulation are more readily done via pedals. But using the guitar's controls affects the sound of the guitar in ways that appeal more to purists, who also like the idea of having fewer components between their guitar's output and the amplifiers. It also means less to own, maintain, pack, transport and unpack, fewer electrical connections, fewer things for stoned performers to trip over onstage, and the performer doesn't have to be where the pedals are.
Some acoustic guitars and electrics have specially configured pickups and other features which require "active electronics". Basically this means that they add power to the signal, usually with small batteries. These aren't particularly common.
No matter what is installed, it all has to be shielded. Otherwise the electronics can pick up stray signals, resulting in weird unrehearsed noises. This is usually achieved with some foil or other metallic lining surrounding the cavity where the electronics are installed.
That's about it for the electrical stuff on a guitar. Commercial ones, anyway - there's no telling what an individual tinkerer might come up with. All that matters is that the signals have the right electrical properties when they leave the guitar.
And what are those? That'll have to wait for the Next Thrilling Installment of Guitar Science!