I would have loved to go through the setup again and see where exactly I got confused. For this purpose I uninstalled Plex and re-installed it. Sadly for the last 20 minutes it only shows:
<Response code="503" title="Maintenance" status="Plex Media Server is currently running startup maintenance tasks."/>
After you create a Plex instance, you still need to go and create a “Movies” library, a “TV Shows” library, maybe a “Music” library, an “Audiobooks” Library and so on. Those are the ones that show on the left panel in the Web UI, TV App etc. It’s how you manage your media in general
During initial setup, Plex will show you a list of “Default Libraries” that have names like “Discover”, “Web”, etc. You can unselect them. You can also remove them from your Library list later on.