Unfortunately I don’t have any specific suggestions for you, just that there is no such thing as the OpenXR runtime. The drivers for each vendor’s device contain the OpenXR runtime for that device (or multiple of them), so if you have multiple devices you might have multiple OpenXR runtimes installed (although only one of them is active at a time).
The situation is similar to OpenGL or Vulkan where the drivers for your graphics card contain the implementations of those APIs.
The UIs that come with the drivers/tools for a device usually have a UI to make that device’s runtime the current OpenXR runtime. Also, SteamVR can show you which runtime is current (under Settings/Developer “Current OpenXR Runtime”) and there are some tools to switch the runtime (see e.g. this thread for some links) that can also enumerate which ones are installed.