When you create an iOS app and minimize it then by default your app will close.
This is very useful during the development phase, the restart of the app triggers Quick Update.
But when going in production, you want your app to run in the background.
In order to get your app to run in the background and remain open you'll need to go to Configure/iOS tab and scroll down to where it says "iOS Multitasking Support" and check the checkbox.
For Android the default is for the app to remain open.
The app will close automatically after 2 hours of being in the background.