where is my imovie project file?

From within iMovie, click on the Projects tab at the top of your screen. That will reveal all of your projects, displayed as icons, in the Projects browser. You can open, play, and edit them there.

The files related to your projects are located in the Original Media folder on your Finder. For example, if you were to select a clip in a project or in the browser and do a File/Reveal in Finder, a pop up Original Media Folder will display as highlighted the clip that you selected. The folder will also show a list of the other clips in the project or event. They all can be played there. However, don't move, delete, rename, or modify anything in the Original Media Folder or you will delete the clip from all projects that are using it.

Unlike iMovie 9, iMovie 10 does not keep a separate Projects folder and Events folder. Instead, the projects and media are kept in one folder in the iMovie Library, bearing the project's name. You can see a list of your projects if you Control-click on the iMovie Library icon in the Movies folder that you show in your screen shot, and then click on Show Package Contents from the drop down menu. You then will see a drop down folder that lists all of your projects and events. However, you cannot play them there because they are only data and codes that the iMovie app uses to assemble the project when you click on the project's icon in the iMovie Projects browser as described above. You must not move, delete, rename or modify any of the content of the Show Package Contents or you risk corrupting your library.

Click on one of the projects listed in the Show Package Contents to open its folder, and you will see an Original Media folder for each project. The Original Media folder contains the media used in the project. The media can be opened and played there, or copies to your desktop. However, as before, you must not move, delete, rename, or modify anything in the project's folder, or you risk corrupting your iMovie library.

-- Rich

The iMovie 10 app itself has a pretty understandable user interface, that in my opinion is simpler than iMovie 9's. Fortunately, there usually is little need to have to access the underlying structure

I found the underlying file structure to be very confusing until I learned how it all fits together. Instead of putting the media in one folder and the projects in another folder, iMovie 10 puts them both in the same folder in one library. Then the iMovie app links to that one library. So you can move the library to other drives and carry a project's codes and media with it. As you have seen from the Show Package Contents, the projects in the library are not independent standalones. It needs the iMovie app to assemble them into a playable project. Thus it is a data based system.

-- Rich