Can I build a auth modal where users can signup with google then redirected to my app to enter required info like username, bio... ?and at the end let them access the app
Yes, but the logic would highly rely on your app itself, not better-auth. Overall this is generally supported by tweaking with core schema, db hooks, other tables (linked with user ids) and custom api routes
but would I still use the better auth cookie for accessing the app or not ? and if yes how ? because the cookie appear just after the auth with google it does not wait the user to give his username or other required info @sebastian