How in different google services like Gmail, youtube pages connected to my account and integrated them.
Here you find all the details about Google's login system: https://developers.google.com/accounts/docs/OAuth2Login
Google APIs use OAuth 2.0 for both authentication and authorization. This document describes Google's underlying OAuth 2.0 implementation for authentication, which aligns with the OpenID Connect specification.
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Note: If you provide a “sign-in with Google” feature, we recommend using Google+ Sign-In. Google+ Sign-In is built on the OAuth 2.0 and OpenID Connect protocols. It supports over-the-air installs, social features, and a sign-in widget on top of standardized OpenID Connect sign-in flows. Google+ Sign-In works for all users with a Google account, whether or not they have upgraded to Google+.