The app runs on my Fossil. I also have a Pixel 3. Have you done this: In the app on your phone, go to settings, and tap Wearables in the integration section. Tap “Use wearables” at the top of the page. In the following list of wearables, make sure there is a checkmark next to Wear OS (on my phone, there is also a checkmark next to None, but that doesn’t seem to cause any harm).
I also remember getting a prompt at some point while I was setting everything up to install an extension on the watch, and there was a prompt to do that when I looked on the watch. Unfortunately, I can’t remember any more of the specifics, but if you haven’t gotten that prompt yet, maybe that’s why you can’ see it on your phone.