Save System: Load remote resources

Hi everyone,
I need to create a saving system for my GameObjects. The issue I’m struggling with is how to load the saved data when some of the objects were imported at runtime by the user.

To provide further context: the application allows users to design a layout of a house (a room), and they can use predefined objects (prefabs) or choose to load external files (in gltf/glb format). I’m having difficulty figuring out how to save and reload objects imported by the user. How can I accomplish this?

Thank you for your help