They are the same viewer. The free Model Explorer app on the Meta Horizon Store runs 0xr.space in a wrapper, for Quest 3 and Quest 3S, with a launcher icon that goes straight into XR. In the app, Add on the dock opens Android's file picker without leaving XR, and Share sends a link through Android's share sheet. In the browser, Add inside XR opens a tray of recent and gallery models, and Share is pressed before entering XR.
The browser needs no install and runs on a Quest 2 and Quest Pro too, which the app does not. Both are free and neither needs an account. A file picked in the app can be up to 2 GB, or 3 GB for one video, and the app checks there is room for it first. In the browser, opening another file of your own means leaving XR and pressing Open.
Search Model Explorer on the Meta Horizon Store, or open Settings at 0xr.space and use Get the app.
Yes, in the browser at 0xr.space. The app is listed for Quest 3 and 3S only.
Not unless you share a model. Files are read on the headset in both.