Do all players interact with the Mansions of Madness app, or does one player act as GM?
The app introduced with the second edition of Mansions of Madness is designed to replace the role of "Keeper", and as such it's best to think of the app as an automated GM.

This means the app is designed in a way that makes it possible for a player to assume that role, and be the sole player interacting with the app and updating the physical board appropriately.