There is no single official Roblox page for codes. Each experience, such as Grow a Garden, Blox Fruits or MemeReef Brainrots, adds its own code system, usually behind a Codes, Settings, Shop or Twitter icon in the game menu. You open that menu, paste an active code exactly as written and press redeem to receive the reward.
Codes are case-sensitive and time-limited. Developers release them for updates, milestones and holidays, and they expire quickly, so a code that worked last week may already be dead. Only use codes from the developer's own channels or trusted trackers, and never enter your password or buy codes for real money.