Check out this excellent video on Electromagnetism. This is the principle that solenoids and relays use to provide the most efficient electrical circuits. It's over 10 minutes long so grab your favorite beverage and have a look.
The principles here will give you a much better idea how components work like a starter solenoid. Following this video is my own on bench testing a starter solenoid which connects the Battery to the starter motor and engages the starter drive with the engine flywheel.
Below is a demonstration on bench testing a starter by simply connecting battery supply to the solenoid positive terminal and the ground terminal at the starter motor.
The switch terminal which is marked "S" is the signal from the ignition switch that energizes the coil in the solenoid and turns it into an electromagnet forcing the contact plate to connect the main battery power to the motor.
Here is an older post on the disassembly & repair of a Delco 37 MT Starter