Simply click on expand arrow with Shift⇧ Shift key.
It
It will expand/collapse everything under it. So if you need ...to
- expandExpand all (under collection) - click on Collection
- expandExpand only specific bone chain - click on Armature
Example with and without Shift⇧ Shift key:
Or use "+" / "-" key (with cursor in OutlinerOutliner Editor) to open / close one level.
More details in Blender Manual.