How to write a android app

Hi, as we are all using this great egate app i am really interested if ppl can share there knowledge on how write a android app, what program to use, what language to code (i assume that for android its java or kotlin)
Also the order how to build a app where you start first the design the functionality and so on,
Also how you ppl learned it and how long it takes to actually bulid a app,

Thanks in advance for everyone

When i decompile a app i get all the xml files and the java files its thousands of files i assume that when writing the app I don’t need to write all of these files, am i correct?

Yes and no. Those apps were likely written by large teams of developers. Also, even a simple Android app has many many files in the source tree, but a lot are generated when you create the app in Android Studio. I would recommend using Android Studio and Kotlin.