Here's a diagram I've drawn of the starter relay, which (I hope) will help you to understand how it works. Basically, the starter motor takes 80-90 amps, so it needs a thick cable. But you couldn't take such a thick cable to the starter switch, so it uses a low current, thin cable, to operate a coil, which pulls in the heavy contacts which are needed for the starter current. When the safety switches are made, and the starter button is pressed, current flows through the coil and draws the main contacts closed. Release the start button, and the contact opens.
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