To keep this basic and not go too technical, the valves are only open to the ports and intake and exhaust passeges. The area where the rockers are isn't open to that area. As to how the oil flows, there are passages cast in the head and block that return the oil to the pan. For filling, you only need to take off the cap itself, the rocker cover only comes off when the rockers need service. These cutaway pics from the manual will help you understand better how the passeges are cast in the head and block.
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