The demo scenes were all from what is in the Examples/ folder. I am working on getting the 3D assets hosted somewhere (some people made some suggestions in this thread I am going to try) and once that is done people will be able to render all the demo scenes locally. All the assets are either from graphics research samples such as Sponza or Bistro or they were CC licensed on a different site such as Sketchfab. I would have tried Quixel Megascans too but I think you have to buy a subscription.
"Even if that’s the case, is there a huge amount of work to integrate existing models/scenes into your engine?"
It uses the Assimp library which allows it to support things like .obj, .fbx and .gltf. Usually what I do is export from Blender to gltf and ask it to export the textures separately rather than pack them into the binary. Most assets that were designed with metallic-roughness in mind either work immediately or with minimal changes. In my own testing the most frequent things I had to change were if normal maps were inverted (fix by flipping the green channel in photoshop) or if the model exporter set all the metallic values to maximum due to missing data (fix by loading the metallic or metallic-roughness maps in photoshop and flipping metallic channel from max to 0).
"Even if that’s the case, is there a huge amount of work to integrate existing models/scenes into your engine?"
It uses the Assimp library which allows it to support things like .obj, .fbx and .gltf. Usually what I do is export from Blender to gltf and ask it to export the textures separately rather than pack them into the binary. Most assets that were designed with metallic-roughness in mind either work immediately or with minimal changes. In my own testing the most frequent things I had to change were if normal maps were inverted (fix by flipping the green channel in photoshop) or if the model exporter set all the metallic values to maximum due to missing data (fix by loading the metallic or metallic-roughness maps in photoshop and flipping metallic channel from max to 0).