What I want is to access the customer details in the client side in next.js and I need to pass the customer access token to the backend API. Using next-auth, I was able to store the access token to the session but is it safe or is it better to use the next.js api route and use getToken function? I'm still a beginner in frontend security, I don't know if I'm just being a paranoid but I can't find a topic/post that states it is completely safe.
So by default the session strategy is set to jwt, which means your session is encoded / signed and safe to store sensitive info in.
However, NextAuth.js can be used to automatically put that accessToken you get from your OAuth provider in the JWT token via the jwt callback. Therefore you can pull it out anywhere with the getToken() method you mentioned and authenticate against Google APIs for further use (get drive contents, contacts, etc, etc, etc).
Check out this example for how to do that in the jwt callback: https://github.com/nextauthjs/next-auth-refresh-token-example/blob/57f84dbc50f30233d4ee389c7239212858ecae14/pages/api/auth/%5B...nextauth%5D.js#L67