The file exported from iMovie will be in the widely used H.264 format and will have the extension. You can move this file to another location if desired (or Trash it if no longer required). The original exported file will remain on the Desktop. This process will copy the movie file to the thumb drive. Finally, drag the exported movie file from the saved location (say the Desktop) and drop it onto the thumb drive's icon (which should be on the Desktop). Insert the thumb drive into one of the Mac's USB ports. Depending on the duration of the movie, it will take some time for the export to complete. Give the file a name and select a location where it will be saved after export, for example, the Desktop. Choose an appropriate size (dimension) from the presets, such as Large, HD 720p or HD 1080p. So, firstly export the completed movie project using 'Share > Export Movie' (from iMovie's Menu).

This action will encode the movie in a suitable format for viewing outside of iMovie. You will need to export the project first. I'm assuming that you wish to view the completed movie on another device that accepts USB drives, such as a media player or Smart TV, or for passing on to someone else in a convenient package perhaps.

How to save imovie on mac