Access to Java code?

Is there any way of downloading the full Java code of my app so I can study how it is built?

Not really. Why?

and How to view Source code

You can download your Project aia of your app and use PKUnzip to open it (the aia is actually a zip file) and examine the files of your app 'source'

Another way of seeing how your app is built is to view all your Blocks. Essentially with this platform, the Blocks are essentially the source code. Also rereading Understanding an App's Architecture might help.