This course provides the foundations and techniques for understanding and modeling the environment in Extended Reality (XR) applications. It covers the use of external SDKs in graphics engines to estimate geometric models using primitive surfaces, as well as user navigation in virtual or mixed environments. Methods are explored for identifying interaction surfaces, handling occlusion, and performing joint estimation of the environment and navigation. Additionally, object recognition is introduced as a key component to enhance immersive and adaptive experiences in XR environments.