If the videos are on your internal drive and you have also used them in iMovie, you will have full sized duplicate files -- one on your desktop and one in the iMovie Original Media folder in the library. So, if you delete the files on your desktop, the duplicate will remain in the iMovie library. The same may not be true if the Finder files are located on an external drive or were obtained from an Event folder of a different iMovie library. Those files might be referenced by iMovie. To be safe there, you would need to do a File/Consolidate Library media to make sure that all files that are used in the projects are consolidated into the library that contains the projects..
You should verify this for yourself before doing any deleting Create a new blank iMovie library and create a project in it. Drag into the project a video clip from your desktop. Then delete the video from your desktop. The duplicate of the video will still remain in the iMovie library. To confirm, select the video in the timeline of the project and do a File/Reveal in Finder. An Original Media folder (that is a Finder folder) will pop-up and it will display the file name, highlighted. That tells you that the video clip is still in your iMovie library even though you have deleted it from your desktop. I just performed this test and that is the result that I got.
I have not tested for files that are located on an external drive and also used in iMovie. Those files may be referenced and should not be deleted until you first do a File/Consolidate Library media and make sure (confirm) that they are consolidated into your iMovie library. Otherwise, you would be disabling all projects that are referencing those files.
As a matter of good practice, it is risky to have any one app, including iMovie, be the sole storage vehicle for your media. iMovie libraries can become corrupted, or future iterations may occur that might make your media inaccessible. The better practice is to purchase an external drive and format it Mac OS Extended (Journaled) or APFS, and store your files and your iMovie library on the eternal drive. Then you don't have space issues. in fact, it is good to have two duplicate backup drives and put a copy of your data and iMovie library on each one. Can't be too safe.
-- Rich
