Your requirement seems a bit off, especially the part where you want the user to get registered automatically after signing in.
But to answer your question generally, you will have to remove the Auth::routes() in your routes/web.php file so your system will not use the Laravel's built-in methods for logging in, registration, and account verification. Afterwards, you will have to create a matching route to catch all requests for logins and registrations, based on the requirements of your 3rd party service.