Discovery

If a permanent has {X} in its mana cost, X is considered to be 0.

As Dispersal resolves, first the next opponent in turn order (or, if it's an opponent's turn, that opponent) chooses a nonland permanent they control with the highest mana cost among their permanents, then each other opponent in turn order does the same, knowing the choices made before them. Then all the chosen permanents are returned at the same time. Next, each opponent in the same order chooses a card in hand without revealing it, then the chosen cards are discarded at the same time.

If an opponent controls only lands as Dispersal resolves, that player won't return any permanent to their hand. They'll still discard a card.