In April, Google announced a groundbreaking technology that could allow Android users to use their smartphones as hardware security keys whenever logging into Google accounts on their laptops or work PCs.
To use it, users will have to have Bluetooth enabled on both their iOS and Android devices, so the Android smartphone can sign and authorize any login attempts for Google accounts initiated from the iOS counterpart.
The only difference in the login mechanism, is who will be managing the login operations. On desktops and laptops, the Chrome browser is used to communicate with the Android phone's built-in security key over Bluetooth using FIDO's CTAP2 protocol.
Because there's no actual Chrome browser on iOS (Chrome for iOS is actually running on WebKit -- aka a customized Safari), for iOS devices, the Chrome browser will be replaced by Google's Smart Lock app, which users will have to install on their iOS devices.