Authentication scheme
The authentication scheme used is Google Sign-in (Documentation). Sign-in lets the user log into the app with their Google account. The client app authenticates with Google and passes the authentication token on to the server. Sign-in is required for use of the app.
Server-side security configuration class
The client app also uses Spotify Sign-in through OpenId. This allows the user to connect with their Spotify account and get their liked songs. Spotify sign-in is required for use of the app.