Open 0xr.space, press Open and choose the .3dm. It is read in the browser by McNeel's own rhino3dm library: meshes, Breps, extrusions, SubD and blocks are shown in their render colours and in the file's real units, so the model can be placed at size in the room on a Quest or an ARCore phone. Hidden layers stay hidden.
Rhino's curved surfaces are drawn from the render meshes Rhino saves with the file. A file saved with Save Small, or never shaded, has none, and its curved faces cannot be drawn; flat ones are rebuilt. Open it in Rhino, view it shaded and save it normally. Curves, text, dimensions and textures are not shown.
They had no render mesh saved in the file. Re-save from Rhino in a shaded view, not with Save Small.
Yes. The model unit is read and converted, so a part in millimetres or a building in metres stands at its real size.
Layer visibility is respected and layer names label the parts, but there is no layer panel, and materials come through as colour and transparency only.
No. It is read on the device, and nothing is sent to a server unless you create a share link yourself.