I'm also looking into it right now.

The code of android apps is compiled to dalvik bytecode which is contained in the *.dex files in the apk. You will not be able to read it. You need to decompile it to something human readable. One of those formats is smali. The xml files (manifest, ressources) are also compiled and need to be decompiled.

One of the best tools to decompile apks is apktool. It works on pc and there is an unofficial android version. The android version includes an editor and is a gui.

Smali is not that easy to read for beginners so on some apps you can try converting it to java using jadx (standalone on pc and included in apktool mobile). The java code will be full of mistakes and will only be partial but will be readable more easily and will give you a general idea of what the code does. This works well only if the code is not completly obfuscated or was written in kotlin.

I suggest learning about android apps before trying to mod them. Learn Java (and or Kotlin) for android. Learn about activities and other things that are specific to android. If you don't understand the concepts used in an android app you will probably not be able to mod apps.