when I do File/Share...File for an edited movie, I get a zero length .mp4 plus 3 other garbled files -help.

When you export a movie to your desktop from iMovie a zero byte temporary file will appear on the desktop. It may remain for a few seconds after the export completes and then it will disappear. However, it cannot be played during the time it displays on the desktop. Is it possible that it was the temporary file that you tried to open? If the full Mp4 shared out movie icon is on your desktop then that one should open fine.

If the above is not your situation, try deleting preferences and then export again. 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 library. Reopen your old library to get back to your projects.  Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.

Finally, not all Mp4 files are compatible with iMovie. It must be Mp4/AAC. AAC stands for Advanced Audio Coding. If none of the above solved it, Try converting the Zoom Mp4 to Mp4/AAC, H.264, with the free download Handbrake. You can get Handbrake here:

https://handbrake.fr/

A simple way to do it is to open Handbrake and do a File/Open Source. Navigate to your video and choose it as the source from the resulting screen. Then do File/Start Encoding. Wait a couple of minutes for the conversion to complete. Then save and import the converted clip into iMovie.

-- Rich