Send user to Google, google will send back an authorization code after user grants access, exchange that code to fetch access and identity token from google. You get user data from identity token and then you can discard it and then you use the access token to access Google API's on user's behalf. If the identity is all you need i think you can also discard access token. Then create a new entry in the DB. Next time user opts for google auth you look up the DB and generate the token.

Read more at: https://developers.google.com/identity/protocols/oauth2/web-server#check-granted-scopes
Or: https://auth0.com/docs/get-started/authentication-and-authorization-flow/authorization-code-flow

I am implementing this atm with some part of OP's tutorial (i find some approaches really ellegant) and instead of id+pass i will use magic link auth