Quick start
The official description starts with the arena: collect Brainrots, then extract with them. Treat a successful extraction as the first goal before worrying about rare finds.
What feeds progression
Collected Brainrots can be placed in a base to earn money. Strength is the official carry-progression lever, while Rebirth is described as multiplying cash.
Keep the first plan simple
The listing also says players can fight others in the arena for rare Brainrots. Learn when you can leave safely before making PvP your whole plan.