Open 0xr.space in Chrome on an ARCore-capable Android phone, import the file, and press View in your space. The camera view opens and the model stands in the room at the size it was authored. The file is decoded on the phone and is not uploaded.
Android has no system-wide equivalent of iPhone's Quick Look for a file sitting in your downloads. Google's Scene Viewer shows a model in AR, but only one that a web page links to from a public URL. For a file of your own, a WebXR page that reads the file locally is the short route.
The inline viewer does, in Safari. Safari on iOS does not offer WebXR, so there is no in-room view there; for a USDZ, iOS Quick Look already covers that.
The browser did not offer an AR session. That usually means the phone is not ARCore-certified, Google Play Services for AR is missing or out of date, or the page is open in a browser other than Chrome.
It is the size the file declares. glTF is defined in metres, so a model authored to real dimensions is placed at real size; a model authored without units can be far too large or too small, and you can scale it by hand.
No. It is read by the browser on the phone and stored there. Nothing is sent to a server unless you create a share link yourself.