After deploying Airbyte on a cloud droplet and docker-compose up, how can I open it? Should the server have a GUI? Because I couldn't open it from server terminal. I was able to open http://serverIP:8000 from my local computer browser but it is not secure I think.

Yeah we don't have a out of box solution but you can read about it here https://docs.airbyte.com/operator-guides/securing-airbyte