Downsteam O2, Does it Control Fuel Trims?
 

I know, I know. Majority will say, "Downstream o2 will only see if the cat is functioning well".

However, on this platform. Especially the stock BRZ, Not tuned. Does the downstream o2 sensor affect fuel trim?

Yes. It is used for lean cruise during closed loop mode (helps improve fuel economy and emissions when you are holding a steady speed such as highway driving)