Do jets and tanks use a key or password?

On a tank, if they want to keep unauthorized people out, they install a padlock on the hatch. A master switch, like on a fire truck, turns the electrical power on and a engine start switch fires the engine.

Well for one, tanks and aircraft generally aren’t just left parked on the curb for anyone to just walk up and steal.

No, they generally don’t. Possibly a key but certainly not a password.

What prevents theft for military aircraft and vehicles is military personnel. And, the procedures with which such equipment as tanks and aircraft are stored, maintained, and prepared to use. There are crews of people responsible for various aspects of maintaining and operating each such tank/plane. Also, there is the complexity/difficulty of operating equipment, and the time required to get it started. If someone is trained to start and use some equipment, and they know the procedures used for readying the equipment and reporting for it, and it is equipment that is ready for use for some reason, there’s an assigned crew and the other people involved know who it is, but if the thief knows all about that, and then manages to convince all the people who see him well enough to identify him that he is an authorized tank crew or plane pilot, then he might be able to detain the real crew and take control… although if he knows all that, he’ll also know how much attention that’s going to get him.

Those sorts of things add up to know one doing it, at least in the military, recently. It has still happened in some cases, for example by the actual crew assigned to a plane, whose pilot then defects to another country who won’t extradite, such as during the Cold War.

For commercial airplanes, there are also various crew and procedures involved, including required preparation. There are crew and staff all around a plane brought up for a flight, and they all have a schedule and so on. Less of them have guns and military training, though, but still, it would probably not be easy, though small airports and small countries with less security might make it easier.

Stealing a private plane would be a lot easier. Especially one regularly operated by a single pilot. Then you just need to convince someone to let you rent a plane, impersonate a pilot the people you meet don’t know, or convince a pilot who already flies a small plane to take you aboard alone.

But even in that case, what prevents theft is sanity. Someone would have to be a major maniac to attempt even the easiest cases I mentioned at the end, because of the attention and consequences it will tend to lead to.

The “Barefoot Bandit”:https://en.m.wikipedia.org/wiki/Colton_Harris_Moore stole a lot of planes and boats with little more than a lot of nerve. Most of that stuff is not locked up very securely.