iMusic into my iMovie?

If you are talking about Apple Music, the subscription service, those songs cannot be used in iMovie. They are DRM protected. The license is only to stream the songs. You are only renting them, so to speak, so cannot acquire ownership by downloading them into an app.

You can use iTunes songs in iMovie, as long as they were not downloaded from Apple Music.

— Rich

Open an iMovie project and click on the Audio tab at the top left of the screen. In the left side bar, click on iTunes (or Music, if using Catalina or later). Your songs should appear in the audio pane. You must first actually purchase an iTunes song from the iTunes website and make sure that it is downloaded and not in the Cloud.

You can also drag a purchased iTunes song from the iTunes website library into an iMovie project.

-- Rich

iTunes songs cost about 99 cents to $1.29. So, you can inexpensively purchase any song that you might need for a particular project. Then the songs will be in your iTunes library for use in other projects as well.

Also, if you look in the Sound effects tab in the Audio sidebar, there are jingles (song bites) that you can use for free.

-- Rich

You might be able to download your Apple Music to iTunes, but the songs still cannot be used in iMovie. Downloading to iTunes will not remove the DRM (Digital Rights Management) protection.

-- Rich

You are very welcome.

-- Rich