Games you install using Xbox Game Pass remain playable once they’ve left the Game Pass catalogue only when you buy them – and you get a hefty discount when you buy games as an Xbox Game Pass subscriber.
When you buy games at a discount, those games are yours to keep. Similarly, any add-ons you buy are yours to keep, whether you buy the base game or not.
Either way, whenever you sign in to Xbox Live, your game data gets saved online. That means, whenever you next play the game—if you buy it, if you play it at someone else’s house, or if it comes back to Game Pass—you’ll be able to pick up right where you left off.