Camera return routing when using a hub

I am routing 6K studio cameras through a studio camera controller (Cat6a) out to a Smarthub 20/20 (SDI) and into a Constellation 4K (SDI). My question is does the camera SDI return feed come out of the Smarthub or direct from the Constellation?
Hoping that some experienced advice might save me running extra cables.
Hoping that some experienced advice might save me running extra cables.