Google expands the ability to use an Android phone as a physical security key to two-factor authentication include iOS devices (functionality launched in April, but only worked with Chrome OS, macOS or Windows 10), as noted by 9to5Mac.
Due to limitations on the operation of iOS, the process is slightly different here: instead of using Chrome to communicate with the device (used for PC versions), the Android phone is connected to The Google Smart Lock app in place.
The net result is that once everything is set up, when you sign in to your Google account on an iOS device, the app is able to ping the Android phone via Bluetooth. Then, users can confirm that this is the one that is trying to connect to the Android device, which allows it to relay this information to the iOS device and allow the connection.
Since the support for security keys for iOS devices has been somewhat limited, the addition of the option to use an Android phone makes the security feature improved much more accessible. Google envisages that the configuration is used by someone with, for example, an iPad and an Android phone, which makes sense. But there is no reason why we can not use that with an iPhone too, assuming you are carrying two phones.
The new feature should be available today on all Google Accounts.