Unity's XR platform aims to provide the tools necessary to achieve the core principle of the Unity engine – "Build once, deploy anywhere" – for VR and AR projects so that you can target any number of different platforms and devices with a single version of your content. In the past, VR developers have been plagued by incompatible device-specific, vendor-specific, platform-specific SDKs and toolkits. Various other "solutions," both open source and proprietary, have had serious shortcomings and limited support. The XR platform architecture has a technology stack that enables direct integrations of multi-platform deep integration, new features, and optimization. The XR tech stack is shown in the following diagram, which has been taken from the Unity Manual's XR Plug-in Framework page (https://docs.unity3d.com/Manual/XRPluginArchitecture.html):
At the bottom of the stack...