The key and keyless systems still fundamentally work the same way. In purely mechanical ignitions systems in cars prior to the 90s, the ignition switch is just

The key and keyless systems still fundamentally work the same way. In purely mechanical ignitions systems in cars prior to the 90s, the ignition switch is just a momentary switch that requires the key to actuate it. Once on, the key however can't be removed without triggering the momentary switch again that tells the car to shut off. That why when you see someone hot wiring a car in a movie they aren't tying wires together, they just touch them momentarily. Since the 90s, most cars rely on transponder systems embedded in the keys. There are various systems from microprocessors with exposed contacts to NFC wireless chips but they work mostly the same. There's still a momentary switch whether it be key based or not but the car won't start unless the transponder successfully authenticates. Since the transponder system is required to start the car, having the key on the ignition switch became redundant and was replaced with just a push button.