Having opened Google Maps on the smartphone side of the Galaxy Fold, the same app — running in a larger form factor to match the tablet — opens seamlessly when the phone is unfolded.
Samsung calls this "app continuity," and the concept is really simple: Anything you start doing on the smartphone side of the phone needs to continue seamlessly when you open it into tablet mode (and vice versa).
In so many words, both the smartphone screen and the tablet screen need to reflect the same computing system. And with the Galaxy Fold, they do.