If you’re not yet familiar with Kotlin iteration semantics, I strongly recommend going through the Conventions section of the Kotlin Koans

Conventions section of the Kotlin Koans?

:google: Kotlin Koans -> https://try.kotlinlang.org/

If I want to catch up Kotlin and step into Android Kotlin.

If this is your first foray into application development, then you should at least go through all the Koans

Understand.

it won’t teach you anything specific about Android, but it should get you familiar enough with kotlin to let you start reading about Kotlin on Android