What is the mechanism behind starting a car?

When you turn the key of your car,a signal is sent to the starter solenoid via cable.Now the starter solenoid draws power from battery and gives it to the starter motor which has a retractable gear attached to it.

The motor starts and the gear messes with the flywheel to turn the crankshaft. The pistons attached to the crankshaft now draws air-fuel mixture (in case of petrol engine) and as soon as enough mixture is sucked, the battery operated spark-plug makes a spark and combustion starts. During the process the flywheel gets enough momentum to continue on its own, the gear retracts and the cycle goes on...

This is the answer true to my best knowledge, hope it satisfies you.

Summit.