[Q] help! trying to overhaul android os

Hey, I'm a comp sci/eng major and I'm trying to get my feet soaked in android and other languages other than c++, ruby, and go. What I would like to do is take the base of android and rebuild it a little bit by adding features to make more use of our phones, somewhat like how the MIUI ROM is. However, I don't know where to start, I've tried looking up on Google about this, but I keep coming across ROM cooking, and that's not what I'm trying to do. Clearly, I need to learn java, and probably few more languages; as well as how android works as an os which I have. If anyone can lead me into the right direction by sending links on how to give android an overhaul, disect apk, ROMs, and the programs I'll need, I think I should be fine. Thank you in advance. Any insight would be ungodly as well.