TLDR: I've been struggling with the new Sign in with Google functionality and especially the part how I can let the user stay signed in. What I understand from the docs is that Google only tells "this is a user who would like to sign in" but basically I would still need to create my own backend to track that user.
Note this question is about the new Sign in with Google functionality, all the guides/questions I can seem to find are about legacy sign-in and this is quite well described here: https://developers.google.com/identity/sign-in/web/server-side-flow
What confuses me most is basically already stated in the beginning of the guide: https://developers.google.com/identity/gsi/web/guides/overview?hl=en#user_sign-in_to_your_site
You'll manage per user session state for sign-in to your site.
User sign-in status to their Google Account and your app are independent of each other, except during the sign-in moment itself when you know that the user has successfully authenticated and is signed into their Google Account. Users may remain signed-in, sign-out, or switch to a different Google Account while maintaining an active, signed-in session on your website.
I understand the basic principle behind OAuth and the part where you have to exchange the authorization code for an access token and you can verify this access token (which is perfectly described in the legacy guide), but this is now only required for OAuth2 in order to access personal data. If I understand correctly this access token can be used as an identifier for a specific session (as alternative to a password or session cookie).
With the new sign in policy you will only get a JWT which identifies the user. Also apparently the only way to get a JWT is as a response when the user clicks the Sign in with Google button and selects the account in the consent screen (which ideally should only occur once).
What I actually want to achieve is that when a user enters the site I want to send a request "Hey Google this user is visiting my site, do you recognize this session and is it still valid".
Maybe I'm thinking way too difficult, but what I just don't understand is how can the new Google Login actually help me remember and validate users?