A couple of months ago, Google officially introduced the ability to use your Android phone as a physical security key to log into websites and services on your desktop using the Chrome browser. The feature works via a Bluetooth connection, requiring that your two devices are next to each other to log you in.
Now, Google has announced that you can use your Android phone to log into apps and services on iPhones and iPads as well, providing an additional layer of security on the platform. Unlike the desktop link, which relies on the Chrome browser to verify your identity, the two-step verification on iOS uses the Smart Lock app, which makes sense if you're logging into these services through apps rather than the browser.
To set up the security key, you'll need to add a Google account to your Android device, and then go to your account's two-step verification settings on the web to turn your phone into a security key. Then, in order to log in to iOS, you'll need to have Bluetooth enabled on both devices, log in using the Smart Lock app on iOS, and then confirm it's you using your Android phone.
Naturally, this sounds somewhat counter-intuitive if you're trying to log in on an iPhone, but it could still be useful on the iPad - or if you always carry two phones with you.