how is voltage applied to starter

When you turn the key to 'start' a low power signal is sent to the solenoid, aka relay. which is just a switch. The wiring in the ignition is not big enough to carry the necessary power. The solenoid has high amperage 1/0 or so cables to it, and when the key 'tells' it to close contacts, contact is made and sent to starter

this will size up

Same way an F16 works-