Hi, Lina1111,
Here is the procedure for transferring an iMovie project to another person for editing on their computer. Create a new iMovie Library. Put your project into it. Transfer the library that contains the project to a Thumb Drive that is formatted Mac OS Extended (Journaled). Send the thumb drive to your friend or colleague. Your recipient will need a Mac computer and must be running the same exact version of iMovie that was used to create the project. Then the library and project can be opened and the project edited on their computer.
To format a thumb drive to Mac OS Extended (Journaled) open the Disk Utility app that is in the Utilities folder in your Applications folder on your Mac. Here's a link that describes how to reformat a USB Thumb drive:
https://osxdaily.com/2012/01/04/format-an-external-hard-drive-or-usb-flash-drive-for-mac-os-x/
Be aware that reformatting a drive erases all data on it. So remove or backup any data that you want to keep before starting the reformat process or transferring anything to the drive. You might want to do a control-click>Get Info on the drive's icon to verify that the drive is not already formatted Mac OS Extended (Journaled).
Once you have the Thumb Drive formatted correctly, you can copy over the iMovie Library that contains the project. I would copy it over rather than move it, so that you keep your original library on your computer.
You can use any correctly formatted external drive. It doesn't need to be a Thumb Drive. However, a Thumb Drive is much less expensive and more convenient for sending to another person.
-- Rich