What a lot of people imagine as separable cultures along borders were really cultural gradients or mixtures (this village here is mostly X, the villages north and south of here are Y, but the village even further south is also X, and we all speak each others’ language).
A lot of our discourse and views on culture are still poisoned by jingoistic nationalism from the 19th and 20th centuries (a cover for othering political foes and motivating the masses to fight and die for political goals), as well as concepts like imperialism and the couple centuries of European political hegemony, plus the elimination of non-prestige dialects/local languages and cultures via mass media.
It’s hard for us to imagine, but in a time before modern concepts of race/organized religion/nationalism the world was quite a dynamic cultural melting pot that wasn’t organized along these more recent distinctions. Greeks traded from Britain to India, pagan religions often saw other cultures’ pantheons as the same religion with different names and traditions, people noticed physical differences between groups of people but weren’t applying racist concepts to them. In a way, people of antiquity were often more tolerant and accepting than many contemporary people. But they could also just decide to kill and invade their neighbors with no pretext beyond wanting their stuff.