Honestly, the easiest way is to simply download the ROM or a full non-delta update from manufacturer, unpack it, and start decompiling. Another option if you a

Honestly, the easiest way is to simply download the ROM or a full non-delta update from manufacturer, unpack it, and start decompiling. Another option if you are able to unlock the bootloader is, using a custom recovery, to create a tarball of each partition and unpack and start decompiling.