Create a web app could be simple, it is just some perl templates to write the configuration file. When you need to create a GUI, then you need to use PHP and dive in Nethgui (the framework for the server-manager)
Obviously the GUI could be harder to code than the template backend…but it is a matter of taste or skill. When I create the backend I think in the same time what I will expose or hide to the user .
The best advice I could give is to read the developer documentation and go to github for discovering some modules until you understand each piece of code and what they did.