Key or switch.

The car and the key do a handshake to be sure that they belong together. If the handshake does not go well, the car will not start. It will not even try. Put the gear selector into any position that is not P or N and try to start the car, the exact same thing will happen if the handshake does not go well.

The ignition switch has contacts inside that will become charred over time, the result being that the contact on the Start Position and starting will become random. When the key hits START, it puts power to the starter to close a relay that in turn provides high power (current) to the starter motor and the huge electromagnet that pushes the starter gear into the flywheel.