I don't think you fully understand how is the process of "cooking" a ROM.



That ROM must be open-source (i.e.: the source code is free to use for everyone), ROMs like Samsung Experience, Oxygen OS, Vibe UI, Color OS etc. are NOT Open-Source, so how to build those ROMs if you don't have access to the source code? Of course, there is a workaround for this, that's the porting of a ROM, but it requires insane amount of work, time to fix bugs and errors. Also worth noting that the device MUST have the same (or at least very similar) hardware, you can't port ROMs for kenzo from, say, OnePlus 5. And even then, kenzo has some old blobs and drivers, so porting ROMs is even more difficult.



Hope that helps.