Imovie does not close after the last update

Try Force Quitting iMovie and then rebooting in Safe Mode. Boot up in Safe Mode and, after everything loads, immediately shut down and reboot in normal mode. Then see if iMovie works properly. One cannot actually open iMovie in Safe Mode. The procedure is done to clear out any glitches that possibly are interfering with iMovie's inability to quit normally.

Start up your Mac in safe mode - Apple Support

You can also try deleting/resetting preferences. To delete/reset preferences open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new empty library. Reopen your old library to get back to your projects. (To reopen your old library close

iMovie and go to your Movie folder or wherever you have your old iMovie library stored. Double click on the library’s icon.  iMovie will open in that library.) Possibly you may need to reboot to set the changes.  Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.

--Rich