This talk presents a modular Swift architecture that works across iOS and Android by separating code into Domain, Data, Platform and Application layers. By writing core logic once in Swift and compiling it for Android, teams can reuse the same business rules on both platforms while keeping UI native. The session explains how modular boundaries improve testability, since shared logic can be tested in isolation without UI or platform concerns. Attendees will learn how to structure modules, how to integrate Swift code into Android projects, and how this approach reduces duplication, improves consistency and supports clean, maintainable multi platform development.