I have imported a variety of tree models from Artstation into blender. When I drag some of these models, the entire object moves together. For other objects, it's like the children are unlinked or something - dragging them with the mouse results in e.g. just the leaves or branches moving. If I select the parent object under the Scene Collection menu, then I can move it as a whole unit.
See the attached picture, the ones moving as a unit (Beech_forest) show triangles in the Scene Collection menu and the ones which seem 'unlinked' (Black Alder1/2) show what appear to be 3D axes.
What's going on here? How can I combine the Black Alder imported objects into one item which moves as a unit when I drag it without selecting it in the Scene Collection?