Now the black alt. wire is positive potential, but it the alt. overcomes the voltage at the Battery, so it back feeds against current flow, towards the battery. And effectively running everything in the car as well as keep the Batt. charged. If a wire/line o the diagrams cross another wire/line it does not connect to it, unless there is a dot at the connection. Like you see on the black wire, changing to red to fuse panel. That is a connection both in the diagram and in the wiring itself.