The result of a Subduel has no meaning or effect on its own on the main duel, unless an effect says so
Ultimate Guard offers various MTG card cases
Strike X SPELL Quick-Play During damage calculation, if your monster battles an opponent's monster: Your battling monster's ATK becomes that opponent's monster's ATK, during that damage calculation only, also, the controller of any monster destroyed by that battle takes damage equal to its original ATK, at the end of the Damage Step
They're marked with a red circle icon when you choose the card in the deck editor, and if that weren't enough of a sign, you can't add them to your deck
This fills your hand with cards, allows you to keep them all, and allows you to cast them all for free as long as it makes it all three turns
And once those two cards are set up things usually snowball out of control for your opponent very fast and they have no chance to ever come back again