It always seems to take me multiple attempts to add a Google Fi eSIM to my iPhone–the instructions in the Google Fi app are tailored to physical SIMs, and the first articles returned in search results don’t seem to work. Having personally gone through the process more times than I care to recall and wasted a lot of time hunting for the correct URL on each occasion, I thought I’d note it here for future reference:

https://fi.google.com/ios/quicksetup

Note that you will need to view this on a desktop OS or you will not be presented with the correct flow. If things are working correctly, you’ll get a QR code which can be used to add your eSIM.