Download the model from Sketchfab in glTF or GLB format, open 0xr.space on the headset or phone, and import the file. It is placed in your room at the size the file declares, and the author and licence that Sketchfab writes into the download stay attached to it in your library.
Sketchfab's own viewer already has a VR mode for a model on its page. Downloading makes sense when you want the model next to your own files, want to cut through it or pull it apart, or need it to keep working offline.
Yes. Sketchfab writes the author, licence and source into the glTF file, and Model Explorer reads those fields on import and keeps them with the model in your library, so the attribution the licence asks for is not lost.
Many published models were not authored to real units. The viewer places what the file says, shows the bounding box size, and lets you scale by hand.
Yes. USDZ is read directly in the browser, on Quest, Android and desktop.