You say that you downloaded Kenney’s assets and the background characters as object files and saved them as meshes. How do you do that? Is it a Blender thing?
Nope! Open the object, then in the inspector click on the floppy disk icon and hit save as. Then save it as a .mesh file!
Though when I opened the project in the new 3.1 alpha the meshes didn’t show up in world, so this might stop working when 3.1 is released, I’m not sure.