Ball vs Ball, by ATYS 3, is a physics auto-battle. You are not steering the ball with a joystick. You pick which ball enters the round, then the arena resolves the hit.
The Roblox description lays out the loop: each round you are offered three random balls, or you spend the one free manual pick you get per match and choose any ball you own. Dropping the other ball to zero HP takes one heart. Matches are 1v1 or 2v2.
Wins pay Coins. The active UPD codes are 100 Coins each, and a single pull from the Coins ball gacha costs more than one code, so redeem the set before you roll. Cosmetic gachas are a separate spend. If you are still filling the collection, the ball gacha is the one that changes the draft.
Rarity is a poor shortcut. Public rankings put Rare balls such as Spider and Vampire in S, and some Legendary balls lower, because the ability has to connect every round. Read the tier list for the reason on each name, then come back here when the draft offers you three bad options and you still have that free pick.