Rule 108.3 · Cards
The owner of a card in the game is the player who started the game with it in their deck. If a card is brought into the game from outside the game rather than starting in a player’s deck, its owner is the player who brought it into the game. If a card starts the game in the command zone, its owner is the player who put it into the command zone to start the game. Legal ownership of a card in the game is irrelevant to the game rules except for the rules for ante. (See rule 407.)
Subrules
In a Planechase game using the single planar deck option, the planar controller is considered to be the owner of all cards in the planar deck. See rule 901.6.
Some spells and abilities allow a player to take cards they own from outside the game and bring them into the game. (See rule 400.11b.) If a card outside that game is involved in a Magic game, its owner is determined as described in rule 108.3. If a card outside that game is in the sideboard of a Magic game (see rule 100.4), its owner is considered to be the player who started the game with it in their sideboard. In all other cases, the owner of a card outside the game is its legal owner.