Custom Oauth Provider with NextAuth, with Nextjs only as the frontend and a custom backend?

situation

• Im using nextjs only as the frontend.
• with a custom java backend.
• with an arbitrary 3rd party oauth authorization server.
• with nextauth.

How do i implement the code flow of the custom oauth provider for frontend?

• IMOPRTANT: I dont use Nextjs on the backend. Its meant for frontend.
• Note:
  Im not asking how to use CredentialsProvider with Email & Password on my backend server (I already had this implemented.);
  Im asking on autentication with a 3rd party oauth authorization server.

All I can think of is following, but I have question on How on some steps:

attempt 1:

• the frontend sends the user to the url of the authorization server.
  • (How the url should be constructed?)
• the frontend gets the authorization_code by redirection
• the frontend sends the authorization_code to the Java backend
  • Do i just send a request immediately when the page is loaded, by useEffect?
• the frontend gets a JWT (access token & refresh token) from the backend by response
• the frontend put the JWT & userInfo inside the user Session of nextauth.
  • How? The code? In [...nextauth]/route.ts?
  • (this is for: the frontend can then use useSession to determine if the user is login-ed,
    also check if token is still valid & refresh if needed in the jwt callback)
• the user session is established

attempt 2:

• the frontend sends the user to a backend url
  • (or receive a redirect url from backend?)
• the backend deals with sending the user to the url of the authorization server, authorization_code, etc (things the frontend wont care)
• the backend redirect the frontend to a page.
  • (For what, passing data or?)
• the frontend gets a JWT (access token & refresh token) from the backend.
  • How? After those redirections, I dont think the frontend has any request-response connection with backend any more.
• the frontend put the JWT & userInfo inside the user Session of nextauth.
  • How? The code? In [...nextauth]/route.ts?
• the user session is established

I dont trust GPT answers, because:

• They always mixing my nextjs into the backend, which I dont want to, because I have a Java backend that should be dealing with security.
• They fail to explain how the function works in NextAuth.
• They fail to explain the redirecting and data flow.
• The answers are insecure, especially after many redirections. After questioning couple more times they either give hacks or denying themselves.

version

"next": "15.1.6",
"next-auth": "^4.24.11",
"react": "^19.0.0",