I have a fridge on bottom, freezer on top style refrigerator I’m going to turn into another fermentation fridge with a temp controller. Will it just change the fridge temperature, or the freezers temperature as well?
It depends. Unless the freezer has a separate temperature controller, it will change the freezer temperature as well (usually to the point where it can't reliably be used as a freezer anymore).
Most top-freezer units have the evaporator in the freezer compartment and have a damper-controlled cold air duct leading from freezer to food compartment. While some units allow some latitude in separate temperature settings for each compartment it only goes so far. One is not going to do 65°F fermentations in the food compartment while keeping hops frozen in the freezer section...