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Here are the Veritasium videos on the exact matter

They are not sailing downwind in this case - they're sailing across the wind, which is why they are able to travel so much faster.

You can sail downwind faster than the wind: https://www.youtube.com/watch?v=jyQwgBAaBag

Sort of a yes and no thing. Once the apparent wind is forward of the beam you are effectively sailing upwind.

This propeller method is a completely different way of doing something similar, but straight downwind.