If you want to advance your Kotlin knowledge and understanding, the best way to go forward is to try the Kotlin Koans.
Kotlin Koans is a tutorial that will lead you step by step from a rookie to a very competent Kotlin programmer in a few days, and even better, it's free.
You can try Kotlin Koans at https://try.kotlinlang.org/#/Kotlin%20Koans/Introduction/Hello,%20world!/Task.kt:
