While resolving Largepox, first the player whose turn it is chooses a card in hand without revealing it, then each other player in turn order does the same. Then all the chosen cards are discarded at the same time.

After each player has lost life, the player whose turn it is chooses a permanent of each listed type. One permanent can’t be chosen twice this way, and that player skips any types of which they don’t control a permanent. Next, each other player in turn order does the same, knowing choices made before them. Then all the chosen permanents are sacrificed at the same time.

After exiling cards from their graveyards and moving cards from their libraries, the player whose turn it is chooses a counter on a permanent they control, then each other player in turn order does the same knowing choices made before them. Then all of the chosen counters are removed at the same time.

A player with ten or more poison counters loses the game as a state-based action.