Both together. The user clicks on the sign in with google button -> frontend should call the backend API (which we expose via our SDK) to get the redirection URL -> frontend redirects the user to the sign in with google page -> user completes auth -> google returns an auth code to your app -> the app calls the backend API (which we expose via our SDK) to exchange the auth code with the user's info, and a new session is created -> user is logged in. Now for reactJS, we have frontend support, so it all happens automatically from your point of view. But for react native, we don't have frontend support yet.