Android differs from iOS in that there is a permanent back button shown on the screen. It is either a hardware button or a software button. It is always in the bottom left hand corner.
One confusion over how the back button works is when you press it and the app disappears and you're shown the launcher (the screen from which you start apps). It is important to realise that this behaviour is by design, despite the fact that it can be confusing.
If you want to know what is correct behaviour on Android a good starting point is one of Googles many apps. Take the case of Analytics - the app Google has released so you have easy access to your Google Analytics properties. This app has a navigation drawer, that is a menu which is shown when you tap the "hamburger icon" in the left top of the screen. By default you start on the Home screen. If you bring up the menu and tap Dashboard you will be taken to the Dashboard screen. Now, tap the back button. It is logical to think that you may be taken back to the Home screen. However you are actually taken out of the app back to the launcher!
Screens in apps can be broken down into two categories, in terms of their place in the hierarchy of screens:
Top level screens - in the case of Analytics this includes Home, Dashboard, Real-Time; for Google Photos it is the screens which are shown when you select a bottom bar item (the Android equivalent to the iOS tab bar) such as Assistant, Photos and Albums. Developers may term what happens when you switch between these as changing the displayed "fragment".
- Other screens which are often used to show detail for selected content. These screens often have an "up" button (looks like the iOS back button) in the top left of the screen. For example in Google Photos if you select a picture the "up" button is shown which can be tapped to dismiss the image, or you can use the Android back button at the bottom of the screen.