The rapid diversification of mobile device form factors, particularly with the emergence of foldable and large-screen devices, has introduced new challenges in user interface design and mobile software engineering. Traditional responsive design approaches, largely based on static breakpoints and screen scaling, are insufficient to address the dynamic and context-dependent nature of these devices. As a result, there is a growing need for adaptive systems capable of responding to varying screen configurations, interaction modes, and usage contexts in real time. This study proposes a framework for engineering adaptive mobile interfaces through scalable design systems that extend beyond conventional responsiveness. The framework conceptualizes adaptability as a system-level property, integrating layout intelligence, component flexibility, and device-aware rendering into a unified architectural model. It examines how interfaces can dynamically reconfigure across multiple display states, including folded, unfolded, and multi-window environments. The proposed approach introduces a layered architecture consisting of device abstraction mechanisms, adaptive layout engines, and component-level adaptation strategies. Particular attention is given to the role of design systems in maintaining consistency while supporting variability, as well as to the integration of these systems with modern mobile frameworks such as Flutter. The paper further explores interaction models specific to foldable devices, including dual-screen coordination and continuity across display states. It also addresses performance considerations related to dynamic layout recalculation and multi-context rendering, as well as organizational challenges in adopting multi-device development strategies. The findings suggest that adaptive design systems provide a robust foundation for managing the increasing complexity of mobile interfaces. By enabling context-aware and device-responsive behavior, these systems support more flexible, scalable, and future-proof mobile applications.