Just check out my explanation of why Rus and Ukraine are totally diferent:
And also a pic in post #35
About Novgorod and other principalities I said here in the “unique mechanics” paragraph:
In short, I propose not to rack your brains over which city to embody as a civilization, but crate Rus’, at the same time there would be a unique mechanic that would allow, without moving into the imperial era, to effectively linger in the castle, choosing which principality to implement, and Novgorod is one of the options there (along with Tver, Ryazan, Vladimir-Suzdal). The transition to the empire means a change of principality to Moscow.
This fully reflects the historical process - Rus’ really lingered too long in archaic forms, but this is precisely what allowed later sub-civs to exist.