The basic thing for you to understand when creating connect apps (either for confluence or lira) is that you are responsible for hosting your app, storing the app data and providing the interface for editing and displaying the data within the context of the host application.
To make sure you create a consistent user experience, It is highly recommended that you look at AtlasKit, the React component set provided by Atlassian that will allow your app to have the same look and feel as the host product.
The only thing the Atlassian Cloud host product dus is create the appropriate navigation that allows users to access your app. For all the other aspects, you’re on your own.