Difference between Events and Projects

Here's is how I have been framing it after lots of excruciating time spent trying to make sense of this. Yes Dorothy we are far from Apple's old intuitive era.
The a) Library holds b) Events - these are your raw footage that you bring into IMovie.

You create a c) Project and give it a name - that can be the same as the events or a different one. You do this by drawing from the Events (and/or from photos) to build your Project, But you can't really open your Library to see your Projects because Projects is simply a series of commands you have given to your Event (move this here, cut this from there, etc).

Your Project is not a Movie however - even though you got to choose in the beginning between "Move" or "Trailer".

It is not until you render it i.e. export it into a format that it becomes a d} "Movie"

So in short the sequence is Event > Project > Movie. They are all contained in the Library but not are visible as if they were a normal series of documents in folders within folders.

The three tabs at the top of the IMovies window actually mirror that reality "Media/Projects/Theater".