iMovie has limited capabilities to do the things that you mention in your post. If your intent is to create a final project that uses only clips unused from previous projects, then you would need manually to view each of the other projects and catalogue the clips used versus unused. Could be a daunting task if there are a lot of projects. Be aware that the same clip may be being used in multiple projects. When you delete the clip from the system, it will be deleted ifrom all projects.
I suppose that at the outset you could put all of your clips into one event, and then delete them. or move them from the event into another event, as you use them in a project. (Delete them just from the Event and not from the system itself. Otherwise you will delete the clip from the project as well.) At the end of the process that would leave you with an Event that contained only unused clips, or one event that contained used clips and another that contained unused clips.
Another way would be to do the file management in the finder. Put your clips into a an "Unused Clips" finder folder and move them to a "Used Clips" folder as you import them into iMovie projects.
For archiving, you can place a copy of your iMovie library on an external drive that is formatted Mac OS Extended (Journaled) or APFS. That will be a back up of your original library. Then you an do anything that you want in your original library because everything will be backed up on the external drive library. Before copying the original library, click on the library's name in the iMovie sidebar and do a File/Consolidate Library Media. That will insure that all media will be consolidated in the library and will carry over and be archived in the copied library.
As for collating used and unused media in iMovie, I know of no way to do that other than as described above. There is no way in iMovie to identify used versus unused media except on a project by project basis. In the project view media screen, all clips that are used in the project will be underlined with an orange line. However, if you delete from the system all of the other clips without an orange line, it would delete them from all other projects that may be using those clips.
I think that the easiest way is to create a new, additional, iMovie library to do your final project. Import into it all media that you want to use in the project. Then archive your old iMovie library by placing it in an external drive formatted Mac OS Extended (Journaled) or APFS (after beforehand doing the File/Consolidate Library media procedure described above). Of course, you would need to manually determine which clips have been unused in earlier projects, and I realize that that is the issue you are trying to address.
The upshot is that iMovie is a movie editing app with some file management capabilities. However, file management is not its strong suit.
-- Rich
