Projects cannot exist outside of an iMovie library. So to transfer the projects to an external drive you must transfer the library that contains them. The external drive must be formatted either Mac OS Extended (Journaled) or APFS. If you need to reformat your external drive be aware that reformatted erases all data on the drive, so you first need to separately back up any data that you want to keep.
Read this link to see what types of drives on which you can safely store an iMovie library.
Move your iMovie for Mac library - Apple Support
The safest procedure is to select the iMovie library's icon and do Edit/Copy. Then open your external drive and do Edit/Paste. Double click on the library icon on your external drive to have it open in iMovie. Verify that all of your projects and data in fact carried over and that the library is functioning correctly. Then you can go back to your internal drive and delete your original library if you want.
Before doing the above procedure, you should consolidate all of the media in the iMovie Library. To do that, select the library's name in the iMovie media sidebar and do a File/Consolidate Library Media. That will insure that all of the media in the library carries over when you copy it to the external drive. Then you can copy the library to your external drive per the above.
You can do the same procedure with the Theater folder as well, except you don't need to consolidate it. However, before doing anything with the Theater folder, read this link:
Move iMovie Theater content to iCloud Photos - Apple Support
As for the iMovie Library.fcpbundle and Untitled.fcpbundle, just leave those alone where they are. No need to move them.
-- Rich