This chapter examines the critical role of UI design in shaping immersive fashion shopping experiences through XR technologies. Moving beyond traditional 2D graphical interfaces, XR UIs require spatial, multisensory, and interactive systems that blend functionality, aesthetics, and brand identity. Drawing from a literature review of XR retail applications, the chapter explores how interfaces mediate interaction, guide navigation, and influence consumer perception and decision-making. It introduces a structured framework for understanding XR UI components—visual, interactive, sensory, navigational, functional, and informational—organized into narrative and call-to-action roles. These elements enable both cognitive engagement and action-based interaction, from product exploration to VTO and checkout. Key design principles include user-centered design, intuitive navigation, real-time responsiveness, and integration with retail systems such as inventory, analytics, and payment modules. The chapter also highlights emerging input modalities like gesture, gaze, and voice, which support natural, accessible, and personalized experiences. Limitations in current literature are discussed, alongside future research directions focused on accessibility, performance optimization, and behavioral impact. Ultimately, this chapter positions UI design not merely as a technical layer but as a strategic enabler of meaningful, engaging, and brand-aligned XR shopping experiences.