Not that, it's about how you navigate through screens in the same app and in multiple apps. Since the back gesture and back stack is universal and all apps are

Not that, it's about how you navigate through screens in the same app and in multiple apps. Since the back gesture and back stack is universal and all apps are presenting activities added to the back stack, you can seamlessly move between apps using "intents" which are abstracted messages that multiple apps understand, and even tunnel through an arbitrary number of apps, and back up through each of their screens as if they were all in the same application. An example of an intent is "can anyone open this PDF file" or "the user wants to pick a file, can any app do that and let me know what they picked?"-- those file pickers could also be from your network browser app, or Dropbox, etc. And since activities are serializable (well, technically the intents that led to those activities being started), Android can do this without requiring the apps deep in the app stack to be running. It can freeze those apps to reserve resources and restart them at the specific activity when the user returns, if necessary. iOS does have limited inter-app linking (it's the tiny little back arrow and the previous app name that you'll sometimes see at the very top of the screen), but since back gestures aren't universal, the only reliable way to activate that feature is to tap it, since the app may not even understand back gestures, let alone do the extra work of relinquishing control when their own local back stack is empty.