If I delete, reject, or whatever the 'project' or 'event' that I can see in iMovie, does this change or delete any of my files stored in the original folders on my Mac? - which are nothing to do with the iMovie system.
No. Assuming you are using iMovie 10, the projects and all the media in the iMovie 10 system are stored in the "Original Media" folder within the iMovie library. (The iMovie library folder is found in the Movies folder on your Mac.) The Original Media folder is a separate folder from any other Finder folders in which you have stored files, even the same media. (You can reveal the Original Media folder by control clicking a clip in the project media, and doing a File/Reveal in Finder. The selected clip and all others in the project will be shown listed in the Original Media folder.) Deleting media from the Original Media folder will not delete them from those other folders. They are duplicate files. However, deleting the project alone will not delete the media in the Original Media folder. To delete the media in the Original Media folder you must also delete from the All Events folder, in the iMovie sidebar, the event that bears the same name as the project you deleted. Doing that will remove the media files from the iMovie system. Consequently, it will also remove them from any non-deleted projects that may be sharing that media with the project that you deleted, and those projects will be disabled.
Caution: Always delete a project by clicking on the project icon in the Projects browser view in iMovie, then clicking on the little circle next to the title, and then selecting to delete the project in the drop down menu that appears. Do not delete projects from the Package Contents folder that you access when you control click on the iMovieLibrary icon in the Movies folder, or you risk corrupting your library.
Similarly, as I have several gig in my iMovie library, can I delete that as well? Or is that going to delete my files stored elsewhere?
I assume that you mean deleting files from your iMovie library and not deleting the iMovie library itself. Don't delete the iMovie library file itself, or you will lose all your projects. Deleting files from the Original Media folder in your iMovie library as described above will not delete your files stored elsewhere. Deleting the Project and the accompanying Event bearing its name will in fact remove the files from the iMovie library. No need to do anything further. If you start randomly deleting files from your iMovie library you will disable all projects that contain those files.
Since space seems to be the issue, you might consider purchasing an external drive, formatted MAC OS Extended (journaled) and storing your iMovie library on it. If you don't maintain a Time Machine backup on a dedicated drive, then get a second back up drive as well, so you keep a copy of your library on each drive. Better safe than sorry.
As for the movies that you have shared out, they are standalone and do not relate back to the iMovie project. Your ability to edit them will be limited, so you should keep the project itself if you expect to make any further edits.
I hope that I have explained it clearly.