Hello,
You’re referring to the AnKing Step Deck, correct? The AnKing Step Deck isn’t divided into sub-decks. Instead, it’s organized by tags, so you’ll need to unsuspend your cards by tag to study them: How to Get Started Studying
You can also create custom sub-decks and manually move your cards between them and the main AnKing Step Deck, as there is no automatic way to do this.
You can learn more about how the AnKing Step Deck is structured here: AnKing Step Deck Wiki.