Extended Reality (XR) technologies are reshaping digital interaction by shifting from traditional 2D interfaces to immersive, always-on, and context-aware 3D User Interfaces (3D UIs) that surround users and dynamically adapt to environmental changes. However, current XR development practices typically require XR UIs to be hardcoded as single, monolithic instances within individual applications. This approach limits adaptability, hinders cross-project reuse, and constrains the support for multiple, modular UI components that can coexist and dynamically adapt within a shared XR environment. We introduce XRSynthesizer, a framework that enables the dynamic assembly and adaptation of independent XR UIs in a spatial UI conglomerate at runtime. Leveraging cloud-based storage and an orchestrator-driven retrieval process, XRSynthesizer decouples UI creation from the host application, allowing developers to register, distribute, and update XR UIs independently of the application’s build cycle. Through a structured four-stage workflow—starting from the creation of a single XR UI to real-time synthesis of multiple independent XR UIs into a unified, context-aware interactive experience— the framework promotes modularity, reduces development overhead, and supports flexible, context-driven XR experiences.