I have created a project using the Windows XR Plugin and XR Plugin management. I am NOT using the XR interaction toolkit, I have created my own tools, including my own XRRig using the Tracked Pose Driver.
My project is NOT built with UWP since I need file access that UWP does not provide or is too tricky to provide (after many attempts and work with a senior developer here, I just gave up). So I'm using standard Unity and my build settings are "PC, MAC, Linux stand-alone". My Player settings in the XR Plug-in management are "Windows Mixed Reality". I am using a few UWP functions for file access within the app.
Now, once the build is built, I obviously don't see it within Steam, but it also does not appear in the Mixed Reality list of applications. I have to start it manually by clicking on the icon on the desktop. It works great, but wtf...?
I know for applications to appear in Mixed Reality they must be built with UWP, but then if this build is neither UWP nor Steam, what is it then? How do I add it (or sideload it) to the Mixed Reality applications that the Windows menu brings up within the cliff house for e.g.?
To answer first question on what type of application is being built if UWP and Steam are removed as target in Unity, this is being built as a PC desktop application.
To answer the second question on how to access this application inside the Mixed Reality Cliff House shell when the application is not a UWP application, this can be launched via the “Classic Apps” pin inside the mixed reality cliff house shell.
Here is more information on the "Classic Apps" pin: https://learn.microsoft.com/en-us/windows/mixed-reality/whats-new/release-notes-may-2019#how-to-launch
That should answer the question asked on how to launch inside of VR experience itself.