From what I understand, OpenXR is a common interface that needs to be implemented by the headset manufacturer. If Apple doesn't do this, OpenXR apps won't work.

Before OpenXR every headset had its own interface. Valve and descendants have SteamVR, Oculus has its own runtime, Microsoft and descendants have Windows Mixed Reality.

Games/Apps targeting these platforms used to have to implement each one of these interfaces separately if they wanted to support that family of headsets. OpenXR shifts the responsibility onto the headset manufacturer, from my limited understanding.

edit: I don't understand why this is being downvoted? If there is a technical detail I got wrong, I'd welcome an explanation rather than Reddit-esque downvotes.