With USDZ content, what's the best way to link to an external website or take users to a product landing page?
If you have your USDZ content on the web you can check out the AR Quick Look functionality for such things at:
Adding an Apple Pay Button or a Custom Action in AR Quick Look
As far as I know there isn’t currently a way to do such a thing directly from a USDZ sent from iMessage, but I can pass that request along.