How do iOS apps know you're logged into other apps?

Question

Can someone please explain it to me like I'm a two year old:How is it, when I sign into Facebook app on iOS, that when I later install Messenger it knows who I am?Similarly, install Gmail, and later GMaps knows it's me.Is this "cookies" at an app level? Cross-app cookies?How does it even work?What else can they see between apps?

orhanerday · Accepted Answer

I'm pretty sure Google uses shared keychain access.
Since iPhone OS 3.0 it has been possible to share data between a family of applications. This can provide a better user experience if you follow the common path of free/premium applications or if you have a set of related applications that need to share some common account settings.
https://developer.apple.com/documentation/security/keychain_...

anderiv · Answer

Apps from the same developer can share information such as login credentials between them. In your example of Gmail and Google Maps, they&rsquo;re both Google-developed apps and such can share account information.

slimebot80 · Answer

I guess these apps report back to Facebook your device ID? So no direct comms between apps?