An iMovie project must reside in an iMovie library. It cannot be transferred independently. To transfer the project to someone else you would need to transfer the library that contains the project. So, create a new library. Put a copy of your project into it. (Keep the original on your Mac.) Transfer the library to a thumb drive or other external drive, that is formatted Mac OS Extended (Journaled) or APFS. Then physically transfer the thumb drive to your collaborator. Your collaborator must be using a Mac and must be running the same exact version of iMovie in which the project was created.
I've not used WeTransfer/Google Drive so cannot comment from personal experience. However, I believe that it will not work because and iMovie Library can only be transferred to a physical external drive.
A possible alternative would be to share out the movie as a file on your desktop. Then you probably could upload it to WeTransfer/Google Drive, depending on Google's size limitations, if any. Your collaborator would receive an independent movie that he/she could import as a large clip into an iMovie project. The embedded edits could not be changed, but some limited editing could be done, such as splitting the clip into segments, deleting segments, trimming the clip, adding music, adding affects, adding titles in some instances.
-- Rich
-- Rich