Who makes the fuel, cabin pressurization, and engine firefighting system for an aircraft with engines on, or in, the wings? Fuel tanks and fuel pipes, pressurized air pipes and heat exchanger, and firefighting fluid tank and pipes are all located inside the wings. However, the controls of these things are in the fuselage/cabin/cockpit. So is the manufacturer of wings--or its subcontractor--in charge of making those systems, or it is the subcontractor of the aircraft manufacturer in charge of these system and their integration? Are there cost benefits to build systems with most of their components inside a section of aircraft (like tail/wing/fuselage/empennage) be manufactured by the subcontractor of the manufacturer of that section?
Usually in modern aircraft the different system's parts are manufactured by subcontractors. There are industries like Safran (see for example the Safran aerosystem part) or aloft that are taking care of designing these parts and propose them on the market.
In general airplane makers industries are assemblers, they design the frame, they define the specifics of the different systems and later they buy the most possible from subcontractors, taking care of the integrations of the different systems. In general this is true for the most of the components and systems when they are not strictly specific to the design and they do not mean a big change of performance with respect to the concurrency.
I can make an example in the jet engine world. In jet engines the injectors are a strategic component that provoke a big change in performance. For this reason it is a component that is manufactured in-house and kept really secret by the jet engine industry. This is a huge difference with respect to the car engine manufacturers, where the injectors are usually bought from subcontractors.