<How do I get an in-depth knowledge of android app...

# stackoverflow

[user]

How do I get an in-depth knowledge of android applications?
I make android apps with java and Kotlin and I can get most of the information about almost any app I want to build online.
I feel like all I do is read, copy and rewrite code with little tweaks. I don't totally understand what I am doing. I only use already built features and implement, extend or override methods.
Is it possible to create an application from scratch without using already built methods or classes?
Is it possible for me to handle things myself, like create my own...