The stimulus starts the fixed action pattern, but that action pattern includes the behavior to make the entire web, not just the start off the process. Different species of spider have different fixed action patterns (and different neurology underlying that, and different DNA underlying that neurology).

I think throwing a ball is more learned, although some behaviors like grasping the ball may provided a fixed action foundation on which higher order learning builds. Baby humans have a grasp reflex that works in a similar way to the fixed action pattern described, but with a simpler action sequence. If you touch the palm of a toddler, they will instinctively tightly grasp. It's interesting that the grasp reflex disappears around 5 years old, so the fixed neuronal pattern is subsumed by higher order learned behavior.