If you find it interesting you can read more about Android system architecture which will answer your questions and give better insight in example how the bootloader is connected to kernel, device tree overlays etc.
However, if you only want to use CoreELEC, don’t bother with Android. Setup everything by following the guide and you should be able to run CoreELEC the way it’s made for 
In addition for better performance, setup How to boot from SD/USB using internal storage for data