Library Visibility: Restrict where content from this library should appear

All settings underneath the server name are server settings and not client settings.

Don’t be misled by the URL in your web browser’s address bar.
This is only the location from where the web app has been loaded into the web browser’s memory.
Once it is there, it runs exactly in the same manner as the one you can load from the local IP address of your Plex server.
i.e. this is not a “web page”, which sends all input and actions back to the address from where it has been loaded. It is a genuine “app”.