By default, iMovie for iPhone adds a dissolve (also called a crossfade) in between all of the video clips in your movie project’s timeline, which is an effect that transitions gradually from the end of one clip to the beginning of another. However, iMovie does not add any beginning transitions to your first video clip or ending transitions to your last video clip. But that doesn’t mean you can’t.
While films can definitely start or end abruptly with no transition, many fade in from black to the first image in the video and/or fade to black on the last image. It’s similar to a dissolve, but… more