Open Model Explorer in Safari on Vision Pro, open a GLB, glTF folder, USDZ, OBJ, STL or FBX, and enter the immersive view. The model appears at the size it was authored and you can move around it, cut through it and pull its parts apart. It is a web page, so there is nothing to install.
One limit to know first: Safari on visionOS supports WebXR immersive-VR sessions but not immersive-AR, so the model is shown in a virtual space rather than over your real room. Headsets whose browser offers immersive-AR, such as Meta Quest 3, place the same model in the room itself.
Quick Look shows a USDZ file as it is. This also opens GLB, glTF, OBJ, STL, FBX and a dozen more, and adds section planes, an exploded view, wireframe and UV views and per-part triangle and material counts, which help when you are checking a model rather than just looking at it.
Not through the web. Safari on visionOS does not offer WebXR immersive-AR sessions, so the view is fully virtual. The size is still the size the file was authored at.
No. It is decoded on the device by the browser, and nothing is sent to a server unless you create a share link yourself.