How is the application's source code embedded inside the APK file?

The APK file contains the compiled source code of the application in the form of Dalvik or ART bytecode, packaged into a .dex (Dalvik Executable) file.

The process is as follows:

1. The source code in Java or Kotlin is compiled into Java bytecode (.class files).
2. Then, using the dx or d8 tool, the bytecode is converted into the .dex format.
3. The .dex file, along with resources, the manifest, and other necessary files, is packaged into an APK archive.

Thus, inside the APK, there is a file (or several files) with the .dex extension, which contains the entire executable code of the application for the Android virtual machine.