A player owns the cards that begin the game in their deck. When a card is played, it enters play under the control of the active player.
A player can take control of an opponent’s card. When this happens, that card is placed in the new controller’s play area. If it is a creature, it is placed on a flank of the new controller’s battleline. If multiple effects that take control of a card are used on the same card, the most recent effect takes precedence.
If a player takes control of a card that belongs to a house not in the new controller’s deck, they can make that house the active house during step 2 of their turn.
If a card that has changed control leaves play for any reason, it moves to its owner’s appropriate out-of-play zone.
If an ability refers to cards that a player “has” in play, it is referring to cards that player controls.