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Apps that have the same signing keys can share code and data: https://developer.android.com/studio/publish/app-signing#con...

It seems that you don't use Android. There's an account system apps can use in the os.

Just checked mine, I can see Google, Facebook, Reddit and quite a lot others there

I am not sure what mechanism is Google using. But Android apps can talk to each other on the same device and you can restrict this to only applications that are signed with the same certificate (and this is guarded at the OS level).

>How does Google share logins between different apps then?

google play services