There's a menu item. I believe it's File > Share > then you choose how (save to disk, email, export to photos app etc).

File > Save doesn't make sense because you would be saving your iMovie project. You're trying to export the final video from the project, not save it.

The problem was using a far too complex tool to do a simple job (and not understanding how the tool works). You can trim a video in Quicktime or Photos which directly manipulates the file.