Hi so you turn the ignition key this makes the starter motor turn which rotates the engine. the electrical circuit provides a spark to the spark plug. which ignites the fuel and air vapour in the cylinder. so the engine fires up. the starter motor is no longer needed it has done what it wassailed to do so stops. the engine should now be running. the alternator is used to recharge the battery which was use to power the starter motor. so next time you come to start the car the battery should have enough power in it to start the engine.

A, the spark plugs.

in our world - a spark plug

What do you think? I mean come on, use your thinking process for a bit.