Google is expanding the ability to use an Android phone as a physical security key for two factors to record iOS devices (the feature that was launched earlier in April, but only worked with Chrome OS, macOS or Windows 10), as noted by 9to5Mac.
Due to limitations of how iOS works, the process is something else here: instead of using Chrome to communicate with the device (as used for the PC versions), the Android phone connects to Google's Smart Lock app instead.
The net result is that once the app is set up, when you log in to your Google account on an iOS device, the app can ping the Android phone via Bluetooth. Users can then confirm that they are actually trying to log in to the Android device, which sends that information back to the iOS device and allows the login to continue.
Since the security key support for iOS devices is somewhat limited, the addition of the option to use an Android phone makes the improved security feature much more accessible. Google foresees that the installation would be used by someone with, for example, an iPad and an Android phone, which makes sense. But there is no reason why you should not be able to use this with an iPhone either, assuming you are carrying two phones.
The new feature should be available for all Google accounts today.