Phone [Q] Custom Android OS

Hi everyone,

So a small group of 6 people and myself are working on a project of getting an unbranded Android based phone to root and update from 4.1 to 4.2 and later.
Updates for a Samsung Galaxy phones for example are released by Samsung, HTC updates by HTC etc.
But an unbranded phone has no company providing updates. Thats where we want to come in.
Android is an Open-source OS so we can download and edit the code, correct?
Or is it a case of downloading the similar OS files for another similar phone?

We are open to trying anything suggested.

Thanks
Tom