A character starts a mission with a deck of 10 cards. The first turn of a mission, each character starts with a hand of 5 cards, but some card mechanics or character traits/skills can alter this. You can play all of these cards during the character’s turn.
Each card resembles an action the character can take, e.g. attacking or moving. All unplayed cards are moved to the character’s discard pile at the end of its turn, then the character draws a new hand of 5 cards. If at any point there are no cards in the deck to draw from, the discard pile is shuffled into a new deck pile, so you will never run out of cards.
You can take up to 3 characters on a mission, each with its own deck of cards.
These decks can be expanded by a) destroying robots, b) finding and opening loot boxes on the playing field, c) visiting the card shop or the black market in the hide-out between missions or d) as a reward of a non-story mission.
Between missions, you can also upgrade and merge cards into better versions or completely new cards. But that is for another post.