i want to change difficulty of AI which part of code deals with it

[jcarolus]

The implementation of the engine is located in https://github.com/jcarolus/android-chess/tree/master/native/project/jni

The UI of difficulty settings is here https://github.com/jcarolus/android-chess/blob/master/app/src/main/java/jwtc/android/chess/play/GameSettingsDialog.java

This is an old fashioned Engine and does not use any real AI, such as neural networks or the like.