There is the Processing code itself (the mode) and there is the PDE, which is the Processing Development environment.
To start contributing to the mode, I suggest first try running examples and become familiar with Processing Android concept and his connection to the Java version. The next step could be tracing really fast the Processing code to understand the structure (check the wiki for the repo structure). I might be wrong, but there is no actual documentation about the code structure. There are docs available about functionality. Now, to make things a bit easier, you should focus only in the PApplet.java file and probably the PGraphics.java file in the current repo. Try to understand how
draw() functions work in general. Then swap to android mode and explore it from the source code. Notice the core resembles the Java version (this is intended). You also need to become familiar with the Android folder as this is the code that lays out the Android structure that all app of this type should have to run in devices and emulators.
If you want to work specifically with the Android SDK and Emulator installer, then my suggestion is that you become familiar with how the actual PDE works. It is a bit ambitious to get to know both the Java and Android version (ok, just focus in the mode, not the actual PDE). This part of the repo should be a good starting point. My suggestion: Try to build the android mode from the repo in your local machine using this and then start tweaking and exploring the code. Focus on how an application is built and package, or at least what commands are used to build the application. Note Processing Android is not implementing any of this from scratch but it is using the tools available through the Android SDK. From what I have seen in the forum, the automatic Android SDK installer doesn’t seem to work for everybody. Maybe this is what it means to “revisit” this topic.
Finally, I invite you to explore previous questions in this forum about Android as it could provide some idea about its users and their challenges. Also visit the current open issues and previous resolved tickets as it could give another insight of the code structure.