This is my understanding:
Think of it in terms of a model plane on a tradmill.
if the plane is standing still, and the treadmill is moving, the plane will not move, the wheels will just spin (ideally). If you take your hand, and exert a foreward force on the plane, it will roll foreward.
This is exactly the same as the engines on a plane pushing the plane foreward. The movement of the treadmill is completely independent of the thrust of the engines.
If it was a car, it wouldnt move…