What does the pilot use? a password? my wife identifies and authenticates herself to her car every morning using her car keys. I don't see the pilot in movies use a similar key?! I don't expect the designers to leave an airplane open for any attacker to go to the cabinet and fly away.
Most smaller propeller planes have nothing. When you're in you can fly away. This seems to be the same for those private jets and older Boeings. I don't fly planes, but I've seen that Airplane Repo program on Discovery, and they just get in and fly away. How this works with the newest planes - no idea.
For smaller aircraft there often isn't anything, but some (especially newer models) do have an ignition much like a car
Some small aircraft and many business jets have an entry key - ie for the doors. It's hard to start the engine if you can't get on board the aircraft! This is especially common for aircraft which are parked at insecure airports.
Commercial airliners usually have nothing, but they are also very difficult to get to, never mind start. If you manage to get to one, sit there for long enough to start it, taxi it to a runway and takeoff..., you'd have taken long enough to alert the authorities and immediately get shot down