Can’t Edit Transitions in Magic Movie on an iPhone
As far as I can see this is not possible. Other answers provided previously refer to iMovie not Magic Movie within iMovie
I see no way to edit transitions in an iPhone Magic Movie.
You can edit transitions in an ordinary Movie project.
A workaround would be to put the two clips into an ordinary iMovie project. Add your transition. Then export it as a file, and then import it into your Magic Movie as one merged clip with a built in transition.
-- Rich