Answer 1

In 2019 Google declared that Kotlin is the new preferred language for Android development.
https://techcrunch.com/2019/05/07/kotlin-is-now-googles-preferred-language-for-android-app-development/

It's not really surprising because Google has a very serious ongoing lawsuit with Oracle, about the usage of Java APIs in Android.
But anyway you can surely use Java as well, if you prefer.