Yes, the disc would be created elsewhere, as I don't think there's a disc primitive in Reality Composer Pro by default. Also, I don't believe there is a way to set the amount of subdivision on any of the primitives in there (you need some high-ish poly mesh in order to modify the geometry and look good). All that being said, you can download the project files for the Diorama scene here and play around with it [https://developer.apple.com/documentation/visionos/diorama/]. The asset in question is the FlatTerrain.usdz.

Hi, you can download the Diorama sample project here. [https://developer.apple.com/documentation/visionos/diorama]

Hi, I'm assuming you've seen it, but Xcode 15.2 beta was released last week and includes Reality Composer Pro. https://developer.apple.com/download/applications/

You might take a look at this session video where she goes over working with content from Reality Composer Pro in Xcode (https://developer.apple.com/videos/play/wwdc2023/10273/?time=1818) including materials (toward the end of video). And the Diorama sample code is available here (https://developer.apple.com/documentation/visionos/diorama), which also might be helpful, specifically the DioramaView where the RealityKitContent is loaded and how they navigate the hierarchy of the Reality Composer Pro scene.