I cut out the subfloor in my half bath on 2nd story above garage. I didn't expect to see 2x4s proving the most support, and just one 2x6 joist. Can someone explain to this newbie what's going on? Thanks.
Those are modern engineered floor trusses, or truss joists. They often have flat 2x3 or 2x4 top and bottom chords and a web of diagonals and verticals between.
The 2x you see is a stiffener, or "stiffback", which ties the trusses together to spread load and minimize movement and vibration.
It looks a bit like someone cut out some of the stiffener, maybe to get a duct in the space below. The builder may have added additional stiffeners nearby.