On the face of it, Processing seems to have very little to do with mobile devices. However, most of the sketches that you have seen so far will run on an Android device with very little modification. Since that is the case, Processing offers a simpler route to mobile application development that many of the standard tools.
To get started with Processing on Android, the first thing to do is to enable the Android mode in the PDE. The instructions for doing this are provided on the Processing web site which can be reached using the top link on the right. In order to work, Processing's Android Mode requires an installation of the Android SDK (API 26). The standard installation should take care of that for you, but if Processing fails to locate the SDK, the solution is to install Android Studio, and then to use the SDK manager to add API 26.
On some operating systems (such as Mac OS), the Android SDK is installed in a protected directory. The solution in that case is to make a symbolic link to the SDK directory, or equivalently to create an alias using the GUI.
The Processing site provides some tutorials - you should work through the one entitled Getting Started. You should be pleasantly surprised at just how easy mobile application development can be...