Apple Motion 3 - Manual (Page 235)

Chapter 2 Creating and Managing Projects 235
For example, when you select a group that has three layers nested within it, the entire
group is selected as a single unit.
Moving a selection in the Canvas moves all three layers simultaneously.
Regardless of how they’re nested, individual layers can always be animated
independently. Subordinate groups can also be animated independently, although
such animation and effects also affect all groups or layers that are nested underneath.
Layers list with group selected Group selected
Before moving group After moving group