I'm not sure why but I had to move my object in the hierarchy in order for it to be in front of another object: Big Rock 3 is on sorting layer Objects_3 and Big Rock 2 is on sorting layer Objects_2 which should be behind layer Objects_3 I thought. am I misunderstanding how sorting layers work in Unity?
It is because your object are located inside a Canvas. Canvas objects are placed according to their position in the hierarchy