The primary functionality of a smartphone's operating system (OS) is to manage hardware and software resources and provide a user interface for interaction. The OS enables communication between applications and the device's hardware, ensuring efficient performance and multitasking capabilities. It supports essential functions such as file management, network connectivity, and security features, allowing users to customize their devices according to personal preferences. Popular mobile operating systems, like Android and iOS, offer unique ecosystems for app development and user experience, making them critical for smartphone functionality.