Open the STL, 3MF or AMF in Model Explorer at 0xr.space and press View in your space on an Android phone or a Meta Quest, then Scale and 1:1 Size. The part stands upright on your desk at the size it will print, so you can see whether it fits the shelf, the hand or the other part it is meant for before spending an hour of filament. The Info panel gives its size in millimetres, its volume and whether the mesh is watertight.
An STL file has no unit. Slicers and CAD tools write millimetres, so a file 10 or more units across is read as millimetres; a smaller one is read as metres, as scanners and Blender write it. 3MF and AMF say their unit and it is used. Print files stand on the Z axis and are turned upright on opening.
The file has no unit, so the size was guessed from how big the numbers are. A part modelled in metres but under 10 units across, or in millimetres but tiny, can be guessed wrong. Scale it by hand; the size readout follows.
Every edge of the mesh is shared by exactly two faces, so it encloses a volume a slicer can fill. A mesh that is not watertight may still print, but holes are worth fixing in a repair tool first.
No. It is a viewer and checker; slicing stays in your slicer. It does open G-code too, drawing the toolpath coloured by walls, infill and support.
No. It is read on the device, and nothing is sent to a server unless you create a share link yourself.