It's a bit of a strange thing to say. At what level? Apple doesn't support lots of formats, neither does Microsoft, people can build tools in their apps to support them, and publish them, and hence new formats are born, that, although may have been created on a platform, the owner of that platform does not 'support' them.

As this is an open format, you can, or very likely, someone else, will write something to parse it on an apple platform, will Apple support it then?

You can convert glTF to USDZ in their Reality Converter app.

Yes Apple does support GLTF, I have a site that renders 3D models that are GLTF format with no issues on an iPad with iOS 13.7. iOS does not support GLTF with draco compression though which is unfortunate but easy to work around by having a separate non compressed model to serve to those devices.

On iOS I would much prefer a second non safari browser than a second app store.