If you set a transition to start automatically, it begins playing immediately after you advance the previous slide.
The easiest way to create a Magic Move transition is to create a slide and add objects to it, then duplicate the slide and rearrange, resize, or reorient the objects on the duplicated slide. Any objects appearing on both slides become part of the transition. Any objects appearing on the first slide, but not the following slide, fade out. Any objects appearing on the following slide, but not the original slide, fade in.
Go to the Keynote app Select slides in the Animate objects on a slide in Keynote on Mac