Hello Wei! The backend ACI is the same container image used in the other steps of the demo, but deployed into the same VNet as the Web App. We deployed 3 of these container groups in the VNet and set up the Web App as an MVC app where the controller can round-robin incoming requests across these containers via their private IPs. We hope that helps to explain things a bit more!
I actually made an ARM template to easily deploy a minecraft server to azure container instances so I never have to worry about setting up a server again. If anyone's interested: github.com/danspark/azure-minecraft-server
