Where are the video clips that appear in iMOVIE stored and how can one transfer them to other apple devices?

The iMovie video clips are stored in the Original Media folder in the iMovie library. The folder is a Finder folder. You can reveal it by selecting a clip in a project or event and doing a File/Reveal in Finder. An Original Media folder will pop up with the selected clip highlighted. From the Original Media folder you can select one or several of the clips that are displayed, and Option-drag (copy, not move) them to your desktop or anywhere else that you want.

Another way is to go to the Movies folder on your Mac (Go/Home/Movies) and Control-click on the desired iMovie Libray icon. In the drop down menu, click on Show Package Contents. You will then see a list of projects and events. You cannot play them there because they contain only data that the iMovie app itself references. However, you can click on the various project and event folders to see within them sub-folders, one of which will be titled Original Media. Open the Original Media folder to find the media that is contained in the project or event. You can then Option-drag (copy, not move) the media files to your desktop. NOTE: Do not move, modify, or in any way change anything in the Show Package Contents or you risk corrupting the iMovie library.

-- Rich

If you have synced your devices with the Cloud you can move any files into the cloud and have them sync. Just remember that iCloud is not an archival storage function. It merely syncs its contents will all devices that have enabled iCloud. If you inadvertently or intentionally delete the synced file on one device it will delete it on all devices.

-- Rich