I want to be an "expert"....

Hi everyone! the purpose of this post is because I have a question about kotlin and java, recently I had a interview about deep tech in java and kotlin, as threads, co-routines and almost how java and kotlin works, so, please, someone can recomend me a book, online course or posts or all of that to be a "expert" in those programing languajes in android?, I really love to do apps but if I just know the superficial concepts to do good apps I never know about how to handle deep problems in memory, garbage, threads, synchronized, etc.

I hope someone can recomend me something.
Thanks!