why is there no music on Imovie?

Music that is purchased through Apple Music cannot be used in iMovie, due to digital rights management (DRM) protection. The songs that you had available on your old computer via iMovie for Mac were those that you had purchased after Year 2009 on iTunes and are not part of the iMovie app but only refer to the songs that you have in iTunes. (You also had, and should still have, the jingles and some songs available when you click on the Sound Effects item.) When you click on the Music media sidebar item in your new iMovie version you should be able to navigate to iTunes to access your previous songs, except those that are DRM protected and those Songs that are stored in the cloud. Cloud stored songs would not be accessible by iMovie until they are downloaded into your iTunes library.

-- Rich

I'm using an older version of iMovie 10 on my Mac running Mojave.

With my version the music can be accessed from the audio pane item at the top of an iMovie project screen. When you click on the audio item the sidebar reveals library items for Sound Effects and Music. You should be able to access jingles from Sound Effects and iTunes for music.

I believe that in the newer versions of iMovie the sidebar item is called Music rather than iTunes.

-- Rich

Adding to my previous post: You should also have the iMovie Theme Music that you can access through the Sound Effects tab in the media sidebar. Those would be the songs shown below. You can find them by scrolling all the way to the bottom of the Sound Effects display in the media browser. That's about all there is from the app itself.

Of course, you can always import songs into iMovie projects that you have acquired and stored in the Finder. For example, you can look in the Finder Music folder and click on iTunes and navigate to the iTunes library on your hard drive. Songs listed in the iTunes library folder can be copy-dragged (dragged while holding down the Option key) to the desktop and from there into iMovie.

-- Rich