Hey @nakura97, welcome to the forums!
The MetaXR plugin is using OpenXR, and you can think of it as an extension of the core OpenXR plugin with more features specific to Meta’s devices. You are correct that some features on the Meta Quest Pro is only available with Meta’s fork of Unreal Engine (such as eye tracked foveation), but many of them can be used with the official branch of Unreal Engine and the MetaXR plugin.
I did a presentation a while back that might help explain some of the questions you’re having: Developing for OpenXR with UE5 | Unreal Fest 2023 | Talks and demos