It seems when you download a new Google Suite app for iOS - whether GMail, Inbox, Calendar or whatever - they offer you to log in with your account and already present to you the account you have logged in elsewhere.
The question is both for iOS and for Android.
I know that apps can use Safari or the new Safari WebView (Chrome Tabs in Android) and a permanent cookie to share data between apps. It's a bit clunky, and can be broken by the user deleting cookies. But other than that, it works.
Google seems to be using something else. AdvertisingID? How exactly do they achieve this feat of sharing data on both operating systems?
AppGroup allows data sharing between two different apps or even app and widgets by creating one common shared path (like document directory). Data saved over there can be accessed by any app which is associated with that particular AppGroup. It is an offline data sharing between apps. Communicating and persisting data between apps with App Groups