800.4a When a player leaves the game, all objects (see rule 109) owned by that player leave the game and any effects which give that player control of any objects or players end. Then, if that player controlled any objects on the stack not represented by cards, those objects cease to exist. Then, if there are any objects still controlled by that player, those objects are exiled. This is not a state-based action. It happens as soon as the player leaves the game. If the player who left the game had priority at the time they left, priority passes to the next player in turn order who's still in the game.
Example: Alex casts Mind Control, an Aura that reads, "You control enchanted creature," on Bianca's Assault Griffin. If Alex leaves the game, so does Mind Control, and Assault Griffin reverts to Bianca's control. If, instead, Bianca leaves the game, so does Assault Griffin, and Mind Control is put into Alex's graveyard.
Example: Alex casts Act of Treason, which reads, in part, "Gain control of target creature until end of turn," targeting Bianca's Runeclaw Bears. If Alex leaves the game, Act of Treason's change-of-control effect ends and Runeclaw Bears reverts to Bianca's control.
Example: Alex casts Bribery, which reads, "Search target opponent's library for a creature card and put that card onto the battlefield under your control. Then that player shuffles their library," targeting Bianca. Alex puts Serra Angel onto the battlefield from Bianca's library. If Bianca leaves the game, Serra Angel also leaves the game. If, instead, Alex leaves the game, Serra Angel is exiled.
Example: Alex controls Genesis Chamber, which reads, "Whenever a nontoken creature enters the battlefield, if Genesis Chamber is untapped, that creature's controller creates a 1/1 colorless Myr artifact creature token." If Alex leaves the game, all such Myr tokens that entered the battlefield under Alex's control leave the game, and all such Myr tokens that entered the battlefield under any other player's control remain in the game.