How do custom updated android roms work? topic

I want to do rom development for my friends unbranded Android phone. I wanna make a KitKat rom for his phone. Im a developer so I have a technical mind and have read some Android kitchen tutorials here on XDA and I was wondering, I know how I would make a custom rom for a unbranded phone by just dumping the rom off the phone in question BUT how would I go about updating this rom to a later version?