The explanation is very detailed and helpful.

jhmoon

This course is a course to learn how to develop Android applications using the Kotlin language. 
You can learn how to use various Android menus and Android components. 
(Lecture materials and source code lesson files provided)

softcampus

Yoon Jae-seong's Kotlin-based Android App Development Part 2 - Menu and 4 Major Components

입문

초급

중급이상

안드로이드

Android

코틀린

Kotlin

Introduction to the course and development environment setup

Menu on Android

Activity

android messaging

Thread and Service

Fragment