I’m on the jump host, How to list or connect to these App & DB Servers
You have to connect via ssh servername -l username
Here are the project details, server details and credentials https://kodekloudhub.github.io/kodekloud-engineer/docs/projects/nautilus
and how to connect to the db server ?
You can SSH into any server from jump host using the correct credentials and hostname/IPs as per given in the document shared above ^^.