You can get the full code from Github : https://github.com/Hype47/droiduino/tree/master/DroiduinoBluetoothConnection

And this is the specific xml file that is related to the code that you are talking about : https://github.com/Hype47/droiduino/blob/master/DroiduinoBluetoothConnection/app/src/main/res/layout/device_info_layout.xml

If you are interested to learn creating Android app for your Arduino project, you might want to take this Udemy course : https://www.udemy.com/course/basic-android-programming-for-arduino-makers/learn/lecture/21287410#overview