Andriod Core Concepts

I started to study the core of android few days back. I feel I am 1 year late to this. Ironically, You don't need to stretch too hard to find it. It is available in Android developer Documentation.
I would recommend you start from (1) Java memory allocation (2) Android Memory management (3) Linux memory management concepts.

1. https://developer.android.com/guide/components/activities/intro-activities
2. https://developer.android.com/topic/performance/memory-overview
3. https://www.youtube.com/playlist?list=PLWz5rJ2EKKc9CBxr3BVjPTPoDPLdPIFCE