The main language to write application since 2019 is Kotlin (https://kotlinlang.org/), it used to be Java.

Apps are written in the prefered language (Kotlin or Java or C or C++ or XML) and then signed using the developer certificate, and then submitted to the Google store as a ZIP file.

Compile:

The app is written in any of the languages (original source code) above and then compiled to DEX (Dalvik EXecutable) which is a bytecode then packaged into APK.

Therefor when we unzip APK we get compiled code which is not human readable.