Mixed reality blends real-world and virtual content into hybrid environments where physical and digital objects coexist and interact.
When you put on your headset, input should automatically get toggled to your mixed reality experience via your headset’s presence sensor.
If input doesn’t get toggled automatically, you will need to manually toggle input to your headset. You can do this by typing Win+Y on your keyboard (and the same to toggle input back to the Desktop).
When transitioning between your Desktop and Windows Mixed Reality, you can choose to use the headset presence sensor to switch input and foreground automatically or manually use Win+Y only.
See also: Troubleshooting Windows Mixed Reality – Enthusiast Guide | Microsoft Docs
This tutorial will show you how to choose to use the headset presence sensor or only the Win+Y keyboard shortcut for Desktop and Windows Mixed Reality input switching for your account in Windows 10.