populations and peoples don’t just suddenly change at a border marker. cultures interact and blend.
there are usually a multitude of cultures in one place, and religions often have different factions within them
what are the differences between the upper and lower classes? is there a lower class? what system is used? fuedalism? capitalism? communism?
how does your society view and deal with poverty?
think about the diaspora. invent a large population of immigrants. why are they there? how long have they lived there? how does their culture now differentiate from their homeland?
languages. is there a global lingua franca (a language that people use to speak internationally. historically this has been latin and french, and right now, english)? if so, why That One? are there smaller lingua francas within different nations?
a large country will almost always have smaller languages within it. put some in. you don’t even have to name them, just have someone mention that they often have to translate for their parents
the lingua franca will usually be the language of the majority, but not always. if a particular ethnic group has control of the government, and therefor education, then that language will probably become more widespread. although sometimes there are “official languages” and “daily languages”
i could write a whole other post about languages honestly
what things do different cultures see as beautiful? is it eyes? hair? what parts of the body are considered scandalous. are the bodies of men and women seen differently? how do people feel about breasts?
how is makeup used? is it daily? is it ceremonial? do different colours and patterns have meanings or is purely for aesthetic? is it seen as gendered?
basically just don’t take everything in your culture as the “norm.” there is no norm. the world is weird.
learn about other cultures in our own world. please.