Public void insert (List<? super Object> list){ }, which objects can be added to the list inside the method insert()?

An instance of Object Class or any of its child classes.
But methods of objects will only be available and the object will be up-casted to Object.

This list can accept any objects and it's equivalent to a non-generic list in that sense.
While retrieving the objects it comes implicitly as Objects and any casting needs to be done manually if required.