There are no locks in doors at all. There is a single mechanism physically connecting the doors together, so that neither can be opened, and that can only be engaged and disengaged from the inside.

Try pointing to the part that is the key that unlocks it, and you will see. (-:

Yes, strictly speaking the headlined article is likely talking about door bolts not locks.

There are plenty of locks that do not use keys, e.g. a keyless deadbolt.

No. The key that unlocks a "keyless" deadbolt is the code that one types in.

That type exists, yes. There is also the variety that is simply impossible to unlock from one side.