Voltage comes from the pickups and the strings. The metal interacts with the magnetic field and disrupts it INDUCING voltage at frequencies the strings vibrate at.
Where the string is placed over the pickup (or vice versa), determines what fundamentlas and harmonics reach full volume or small/no volume. Imagine a pickup directly under the second harmonic's center node. You now have full voltage for the fundamental frequency and as close to zero voltage as makes no difference for the second harmonic. The fourth and sixth will also be similarly of low/no voltage, and 3rd, 5th, would be full voltage.
Knowing this, you'd want the pickup to be offset slightly from center to try to 'capture' all the frequencies at some relatively equal voltages, for a 'full' sound.
On the other hand, say you want a pickup which loses some harmonics, you place it in the spot to lose those harmonics, and place a second one in a position to get/lose the opposite harmonics. Now you have both pickups producing opposite values, and this allows you to select either the former or latter for different tonal qualities, or blend them together to get all of it.