Good question. Each plane is very different, depending on the type of engine.

Small aircrafts (Cessna) tend to have keys, since they are easy to access by an unauthorized person. They’re just like car in this case.

But commercial airlines/jumbo jets don’t have a key for ignition, don’t have a lock, they use switches/knobs and indeed the start-up sequence is complex when compared to small planes.

Likewise for smaller planes, you need keys to secure the exterior doors/cabins/luggage compartments whereas in jumbo jets you need a jet bridge to board the plane.

Ceremonial handling over key for jets used to be carried out, which is just for a ceremony but no key is necessary for airliners. Also the commercial jets are parked in airports which has 24-hour armed security.

Smaller propeller planes have throttle locks as shown below:

These metal devices fit over the throttle lever or vernier control and are secured with either a key or a numeric code.

Source – Throttle Lock