Google updated its Smart Lock app on iOS to let iPhones be used for two-factor authentication. But it will only work inside Chrome. Now your only choices for Google two-factor authentication are this Smart Lock app, or a phone number (an insecure method). You can also use a physical security key but not an app like Authy.
After installing the update, users are asked to select a Google account to set up their phone’s built-in security key. According to a Google cryptographer, the feature makes use of Apple’s Secure Enclave hardware, which securely stores Touch ID, Face ID, and other cryptographic data on iOS devices.
Update. So I made a mistake and you can use an app like Authy, but you first have to surrender your phone number to Google. Which I’m obviously loathe to do so I use a disposable number.
Check It Out: Google’s iPhone Security App Keeps You in its Ecosystem