To simply see how you can build an app using an OpenId provider as the main authentication authority then just go to your Google Api Console and create a new app and a new set of OAuth credentials.
Copy .env.example into .env and fill the values with what Google gave you.
Don't forget to add URI and redirect URI in your Google Console,
the ovbious thing you need to add is http://localhost and http://localhost/auth/callback.
And then you simple yarn start.
If you want to see how mutliple different apps (OAuth Clients) can achieve Single Sign On
using a single OpenId provider such as google (or your companie's internal one) then you need
to create a second OAuth client and get a second set of OAuth credentials, and now you are going to fill
two .env files, one is .env.client1 and the second is ..., you guessed, .env.client2, the format is still
the same.
And then you run two instances of the same code as two separate Oauth clients: yarn start:client1 and yarn start:client2.
To really observe that they are complete distinct in terms of how the store session cookies and such you need
to ensure they run in different domains because otherwise the cookies will collide in the same domain localhost without
the port, one nice way of achieve that is runing one app in localhost and the other in 127.0.0.1.