One of the best things about developing on Android is the availability of the source code of the operating system itself. It means that developers don’t just have a blackbox to work with as with other operating systems.
Downloading the entire source code for the Android operating system can be time consuming depending on your internet connection, and building times depend on your computer’s processing power. For the typical develop who is just curious about the code in general without having to commit to downloading the whole thing, Google has made most of their code web viewable.
Of all the code that have been made available, it is platform/frameworks/base that contains most of the relevant code for most Android application developers.
Another place to look is in everything preceded by packages/apps, which translate to applications compiled as APKs and deployed to the system partition. Taking a quick glance at the manifest file and string resources will help figuring out which packages do what. Some of these packages use hidden APIs unsupported by the Google distributed SDK, including the Launcher application. It is still possible to build these applications into work apps as described here (Stackoverflow).