What file do I need to add a 3D model, and why does AR not work on iPhones?

In Plugins → 3D Customizer, click + Add 3D model, pick the Product and upload the 3D model file as GLB or glTF. That file shows on the product page and is used for AR on Android.

iPhone AR (Apple Quick Look) needs a separate USDZ file. Without it, iPhone shoppers can still rotate the model but won't get the AR view. Tick AR enabled to show the View in your space button, and add a Poster image to show while the model loads.

Each product has one model, so adding the same product again replaces its model.