Finalize project in imovie

After you share out (export) a movie the project itself will still remain in case you want to make further edits and share out again. If, after exporting, you want to delete the project, go to the project browser screen (where your projects are displayed as icons. Move your cursor over the icon and that will cause a little circle with dots in it to appear to the right of the title. Click on the circle and choose Delete Project from the pop up menu.

Deleting the project will remove it from the project browser but will not delete the media in the project. When you delete the project iMovie will create an Event bearing the project's name, in which the project media will be placed. To delete the media permanently from the iMovie library and the Finder, you must delete the Event after deleting the project -- delete the event itself, not just the media contained in the Event. Be aware that deleting the media will result in disabling all projects that might be referring to that media. Also, unless you have duplicates of the media additionally stored elsewhere than in your iMovie library, you will permanently lose your source media.

An iMovie project contains no media itself. The thumbnails that you see in the project are editing thumbnails that refer to the original media that is contained in the iMovie library. So, deleting the project without doing the additional step of deleting the media will not free up significant additional space on your drive. It is the media that takes up the space.

There really is no purpose, other than reducing clutter, for deleting a project unless you also want to delete the project media as well.

Here's a link to Apple's online Help link that describes how to delete a project:

https://help.apple.com/imovie/mac/10.1/#/mov79fa23112

-- Rich