To clarify - there are a couple of things:
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The OpenXR runtime, which is the system component that enables OpenXR applications on your system - this is a mandatory component - you get through your VR system vendor (if you have a WMR headset, the Microsoft store). See FAQ here: OpenXR for Windows Mixed Reality - Release FAQ - Virtual Reality (VR) / General - Microsoft Flight Simulator Forums
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The OpenXR Toolkit is an add-on program that adds additional features to your system - this is an optional component - you get it from our website (see message above).