Sign in with Google is Google's login service for websites and apps: users sign up and sign in with their Google Account instead of a separate password. It is part of Google Identity Services and is built on OAuth 2.0 and OpenID Connect, open standards in which an identity provider, here Google, confirms to another site who the user is.
A developer creates an OAuth 2.0 client ID in the Google Cloud Console and configures the consent screen, which tells users which app asks for what data. The website loads the Google Identity Services client library and shows a personalized sign-in button, a One Tap prompt or automatic sign-in. After the user consents, Google posts an ID token (a signed record of the user's identity) to the site's login endpoint; the server verifies it and stores its sub field as the user's unique ID. Users share only their name, email address and profile picture. SDKs cover the web, Android and iOS; Google says it does not use this data for ads.
A separate authorization API gives access tokens for Google APIs such as Drive. The pages state no price.