So how does this work exactly? How could you "sell" a digital artwork that's apparently freely available to everyone according to this thread, because just this

So how does this work exactly? How could you "sell" a digital artwork that's apparently freely available to everyone according to this thread, because just this thread has two different links to said video? What is it exactly that changed hands then? What exactly does it mean to "own" a digital artwork? I don't understand it. edit: this explanation https://niftygateway.com/whatisanifty https://niftygateway.com/whatisanifty doesn't make it any clearer.