If you have video recorded in the wrong orientation, you can rotate it after you add it to the iMovie timeline. You can also change the original file by opening it in QuickTime Player, rotating it, and saving a new version. If you need to rotate video in Photos, you'll have to export the video, rotate it in QuickTime Player, and then import the new version back into Photos.
Want to know more about how to use iMovie on your Mac? Check out this MacMost course!
Really good tip Gary! - thanks. Hence my Patreon subscription - others should join.
Bob: Thank you!