How do games have logins for facebook/google play etc but have their own database?

I'm real confused how they link the two?

If a play logins in with google play or facebook or apple id how do they then link it to their own database which has player data?

I have no idea where to find this information properly at all.

 

It is easy, if you take a look at their core and understand how they work at their core.
When the user Authenticates with Service Provider, the Service Provider (Googe/Facebook) will provide an Access Token for the user, that means the user can always access the Service Provider Open API.
The Access Token is like a Visitor Card that.

Here are some more resources to help you learn more:

• Dancing with OAuth: Understanding how Authorization Works
• OAuth
• OpenID

Thanks.

 

Indeed. Web APIs and OAuth is something we have been supporting for a while now. Even though there are standards, each API still likes to do their own thing.

http://unity3dassets.com/oauth-brief/