Question about political boundaries and countries in a post-revolution world

In a world where we've moved past capitalism, maybe we still havent achieved communism yet but we have socialism, how would countries work? Like the straight line borders of colonialism would have to go, but what would replace them? Would it be divided on resource boundaries like water?

How would ethnic issues be dealt with? Dividing people on ethnic lines i feel would only increase the division, but if you mix everyone up then you lose all the interesting elements of their history and their people.

My question is, how will stuff be divided up? I assume we wont just have a "one world government" or whatever, at least not in the early stages, so how would we run stuff? what would the political divisions be? would solidarity maybe be focused on class instead of race or ethnicity, doing away with nationalism?

idk, im really just trying to learn what people think and what seems to be the best solution. sorry if i maybe said anything dumb, im not trying to be. thanks for answering!