I see.
You want to find out for yourself that Android graphics are not handled only the same way as iOS graphics and neither necessarily is video.
Fair enough.
You'll find a lot at developer.android.com and source.android.com
As far as learning on a cheap device, you're probably better off starting with the emulator on a pc.
From there you can look into software vs hardware decoding and neon support.
At that point, to save money on an entry device with enough processing power to represent the threshold for more advanced device capabilities, I'd recommend one of entry Motorolas, the Moto G or E.
While working on your project, you still might want to check out VLC. It's very likely going to do most all of what you want.
Unlike iOS, Android is not one thing with rigid rules and most of what you need to root tweak iOS for - when possible - is usually accomplished with a non-root app, including the entire ui/ux.
Best luck with your goal, it sounds very interesting!