I kinda wanna copy how the “Two Tabs” mode of the Project View displays FBX files, with the right arrow to expand a while lot of inner assets inside the fbx. Is the source and UXML for that anywhere to be seen, or it’s proprietary?
You can find the implementation in the reference source:
https://github.com/Unity-Technologies/UnityCsReference
The project widow is unfortunately not using UIToolkit. To see if an editor window is using UIToolkit, you can open the UIToolkit debugger by clicking on the … icon on the upper right. You’ll then be able to inspect it’s content.