Why is it that logging in once with a Google Account allows access to all related services such as Google, YouTube, and Drive, even though they have different domains?
Where does Google store user login information in the browser to provide this seamless login feature?
Can I build a similar system, using a common login system for multiple websites (with different databases) of mine? Please provide some keywords for further research on this topic.
The login system I implemented works as expected within the same website, but I couldn't figure out how to share the authentication session across multiple websites with different databases.