Apple iMovie only has a 16:9 export ratio. You can work around that.

For those that are recording video in 9:16 aspect ratio on your iPhone or iPad, there is a fairly simple solution. Your videos are typically saved in your Photos app. You can open each video in the Photos app. Select the edit icon and then select the crop icon and then select the flip icon. Flip until the top of the video is on the right and select done. Do this to any other videos you want to import into iMovie. Open iMovie, start new project, select movie, and select movie clips from Photos in the order you want them displayed. It will display the videos on their sides, but you can still edit the videos. Complete editing the movie like you normally would. Select done. Rename it, if you want. Then at the beginning iMovie screen, select your project and select the icon to export and share. It will save it back to Photos. You then use the edit function in Photos, selecting crop and then flip the image upright. You now have your movie in 9:16 aspect ratio. It shows the full length of your video ( top to bottom) with no black bars on the side. 😁 Sounds harder than it is. I hope this helps.