Graphics and rendering in an XR app follow the same principles as they do in any Unity app, with some differences arising from the need for stereo rendering and increased efficiency.
XR apps generally require very high and consistent framerates for user comfort. At the same time, rendering in stereo means that every visible object must be drawn twice from different perspectives. Techniques like single-pass rendering can improve rendering efficiency by reducing the duplication of effort, but also require changes to shader code.