Turn off automatic transitions in iMovie on Mac
Some project themes add opening titles and transitions as you’re editing your movie, if automatic content is turned on in project settings. To add manual transitions to your movie, you must first disable automatic content.
Disable automatic content
In the iMovie app on your Mac, click Settings in the upper-right corner of the timeline, and then deselect the “Automatic content” checkbox.
After you turn off automatic transitions in your project settings, transitions are not added to your movie automatically when you add new content, but any existing transitions or titles remain.