Hi, CWALK125,
You cannot import Apple Music into your iMovie project. Apple Music is DRM (Digital Rights Management) protected and cannot be used in iMovie. Also, it is a subscription service that requires the subscription to be maintained in order to listen to songs for personal use. One does not own the songs but is only renting them, so to speak. It is intended for personal listening, not for downloading. If one could download the songs to a project, or to any other app, that would be appropriating ownership, which would thwart the intent of the Apple Music service. You can, however, import songs purchased from iTunes. Songs purchased after Year 2008 are free from DRM protection.
The song tracks that are currently residing in your iMovie app under audio and video, are songs that are located in the Finder -- either in the Original Media folder in the iMovie library or in your iTunes Media folder or possibly your iMovie Sound Effects folder. iTunes songs are located in Downloads/Music/iTunes/iTunes Media. From iMovie you can see where the sound clips are located by selecting the song clip and doing File/Reveal in Finder. When the Finder Folder pops up you can delete them from the Finder in the normal way. However, I would not delete them because doing so would remove them from the system and disable all projects that might be referencing them. Also, many of the clips will be in the iMovie Sound Effects folder or other sound effects folder, and by deleting them you would permanently remove them from your Mac and from any projects that are referencing them.
So, it is highly risky to do any sort of mass deletion, or even individual deletion, of audio clips shown in iMovie. Best to leave them alone. You don't want to be deleting sound effects or songs that might be referenced in multiple projects. Of course, that would be your call.
Note that the iMovie app itself does not contain any media. It just references media that is located in the Original Media folder in the iMovie library, or in some other Finder folder, or in the iTunes library. That's why the iMovie app itself takes up relatively little space. For example, if you delete a song from a project, by selecting it in the timeline and pressing Delete on the keyboard, you would be removing it from the project but not from the Finder. So you wouldn't be saving any space.
-- Rich