How to make mod applivations in androif

Answer 1

*How to make modifications in android application?

-> Reverse Engineering

first
You'll need to have knowledge of android assembly language

After, you can use various tools like

apktook-> to modify Smali code

For source code inspection only:
winrar -> to extract apk
dex2jar-> to decompile classes/2.dex
jdgui-> to read java files in jar archive