Android OS applications executables are typically stored in its own format, optimized for more efficient execution in Dalvik virtual machine.
This format is loosely similar to Java .class file format and generally holds the similar set of data: i.e. classes, methods, fields, annotations, etc.
This page hosts a formal specification of Android Dalvik VM executable (dex) using Kaitai Struct. This specification can be automatically translated into a variety of programming languages to get a parsing library.