So I'm trying to learn rigging/armatures and I've been dissecting the rigify rig to get a better understanding. But some bones seemingly don't have any children parented to them, yet they still deform the mesh. For example, the torso bone moves the entire torso, but none of the spine bones are parented to it. Also I see no drivers or bone constraints that point to it. Any insight?
The deform bones are the only bones that actually deform the mesh. Their name begins with DEF- and they are on layer 30. Almost all of them are green-colored. This means they have constraints. The spine deform bones have Copy Transform and Stretch To constraints with the tweak spine bones (layer 5) as their target.
The torso control (layer 4) has some children and grandchildren. If you follow the hierarchy in the outliner, you will find the tweak bone (tweak_spine.003). The neck controller is the parent of the other tweak bone (tweak_spine.004).
Conclusion: the control bones (torso & neck) move the tweak bones, and the deform spine bones follow the tweak bones due to constraints.