APK

Java/Kotlin code is converted to *.class using javac/kotlinc compiler

Then using another compiler (d8) it is converted into classes.dex

Then this classes.dex i.e. dalvik bytecode is run in Dalvik Virtual Machine on older android versions and newer android versions use ART (Android Runtime)

And Android Runtime makes it little slower to install but improves performance

Basically an apk file is a zip file which contains the above mentioned things.