You can go to https://monsterwriter.com/ and see the system in action. When you click on login you will be redirected to a login provider, which in case of MonsterWriter is auth0. But it could also be an open source (e.g. KeyCloak) or any other commercial OIDC provider. Then it is up to the OIDC provider to display the "login with Google"
The hello world example on the website provides a dummy OIDC provider, If you try out the hello world example I guess this is the best place to really get an idea: https://linkedrecords.com/getting-started/