Automatic parenting of new objects
When a new object is created (by dropping a block, or by pasting the object) a set of rules are evaluated to determine the parent of the new object:
If there is an object selected in the scene:
If it is a container, then the new object will be added to it.
Else, if it has a parent container, then the new object will be added to the parent.
If the scene is a prefab scene:
If the scene is empty, the new object is added to the Display List.
If the root object is a container, then the new object will be added to it.
Else, a new container is created and the old root object and the newly added object will be added to the new container. So the scene will have a new root object: the automatically created container.