For a more thorough/high level overview of the entire WebXR stack, please refer to components/webxr
Chromium's WebXR implementation makes use of the multiprocess architecture for added security. Thus all code which directly interfaces with and talks to the XR hardware (“runtimes”) ends up loaded/hosted in a separate XR Utility process, except on Android where this is not possible. The XrDeviceService serves as the entry point for the VRServiceImpl in content/browser/xr to talk to this process, while the IsolatedXRRuntimeProvider is the main in-process entry point. The runtime provider continually polls for supported runtimes and when a change is detected creates and returns the appropriate runtime over mojom. The mojom interfaces used by this process are defined in device/vr.
This folder also defines test hooks. Tests can set this to modify the behavior of the IsolatedXRRuntimeProvider and force it to create a fake XR Device which the tests can fully control.