Jetpack Compose is the future of Android UI and is Kotlin only.
Insofar as bringing it into the app - from most apps I know of, new code is usually written in Kotlin, and the Java code becomes legacy (which is sometimes rewritten in Kotlin). It works well enough, compatibility with existing Java was important to the Kotlin team (for adoption and other reasons).
Read the Kotlin docs for more https://kotlinlang.org/docs/jvm-get-started.html . You definitely see some of the things mentioned when using both languages, like the JvmStatic annotation etc.