how to download music to use in iMovie?

Music from Apple Music is DRM (Digital Rights Management) protected and cannot be used in iMovie. It is a subscription service for personal listening and not for use in any other app. You would need to purchase your songs from iTunes, or from some other audio website, and download the songs from there. Also, you can drag into an iMovie project any song that you have on your computer, except songs that involve Apple Music in the chain of purchase. You can also use any of the jingles that are contained in the sound effects item in the iMovie project sidebar.

-- Rich