Do planes have keys?
Small planes tend to have keys, but not all.  The bigger the plane the less likely a key is needed and once you get into jets, I can't think of any that need keys for engine starts.  The starting and securing of engines with no keys comes down to switches and knobs.  

Likewise with exterior doors, on smaller planes including private end executive jets, you can find keyed exterior doors and luggage compartments.  With transport jets at airlines you won't find keys. To secure those doors the jet bridge is pulled away from the airplane so there is no physical access to the boarding door.