Note: In your terminal, you’ll see a few log lines when you open/connect to your forwarded port:
Starting session with SessionId: botocore-session-1579056167-0c76865253a1232e
Port 9521 opened for sessionId botocore-session-1579056167-0c76865253a1232e.
Connection accepted for session botocore-session-1579056167-0c76865253a1232e.
And there you go. You can now see the data in SQL Developer !