Library, event, project - and 'leave files in place'
'The highest level of structure is your library.'
A library is the top-level container. Events sit inside it and hold imported footage, auto-named by creation date. Projects are timelines and live inside an event. The consequence people trip over: media in an event is available to every project in that event, and completely invisible if you switch to a different event - which is why footage 'disappears'. Two setup decisions matter. Save the library file in the same folder as the raw footage so the two travel together, and delete the untitled library Final Cut opens with. And on import, choose 'leave files in place' rather than 'copy to library', or Final Cut duplicates every clip and doubles your disk usage for no benefit.
The structural model that makes Final Cut make sense instead of feeling arbitrary.
The highest level of structure is your library.