wtf is "Generic White People" culture? There is no "one culture" to rule them all in America. What we have here is a big television screen, both metaphorically and literally, that everyone is watching. Everyone is in a giant movie theater all together watching this film, it's a very large theater, with an enormous audience, which is the entire world. Each person is sitting in this big audience and is watching what he or she sees on the giant screen in front of them. The screen has a lot of things on it, too many to count, too many to pay attention to, so each person has got to focus on parts of it. A person sees something that he or she likes, and he or she focuses on that part of the screen, and maybe some other parts. That person is also sitting with his or her friends, and they are all talking about what they are seeing on the screen with each other. The big screen is everything that happens in the world, public and private, mostly public. Everyone is mashed together with the same exact influences on their culture and behavior, it's just that some of them can't see other things. Simple example: an affluent American man does not see hunger and poorness on his part of the screen, for whatever reasons he has not to see them, doesn't matter, it is not a part of his culture because of his specific personal things that he sees on the screen. At the same time, he is talking to his neighbors about his specific culture, and this culture mingles together with their cultures and it all becomes a big culture of rich people. However, the rich people are not all the same, they are not all as rich as the others, they are all not ignoring hunger and poorness in the world. This brings different viewpoints to the little collection of people in the audience, a woman a few seats over from the man sees a homeless family on the street and brings it up, it now influences the man's culture.
Basically, culture is a giant web, there is no single culture with a firm, concrete, definition. Regardless of how it "appears" to you, it is this way. A lot of people may be watching the same part of the screen at any given moment, but there are still an infinite number of parts of the screen that are being observed by an individual that some other individual is not observing. This creates diversity and it is there, it is everywhere. Now, this giant web of culture must be generalized, why? I dono, it just should be because then people in other countries can make statements about America even though it has a population of 300 million, or so that I can make generalizations about China even though it has 1.3 billion people.
This giant web can also be seen geographically, as someone pointed out. A town in medieval France is going to have its own little web of culture that is distinctly different from a town four hundred miles away in like Poland or some place that is four hundred miles from France. But at the same time it is going to have many facets that are 100% identical because we're all human beings and that's just the way things are sometimes. What you said about how mercenaries and traveling folks spread culture is true, sure, yeah, to a point. All they do is cause some guy in Constantinople to have a slightly similar vision on his screen as some guy in Edinburgh, but in a way that is so subtle and ridiculously tiny that is impossible to define as something on a wider "Byzantine" culture scale, or whatever, because you cannot say that a couple of mercenaries affected the culture of an entire nation. Actually you can because culture is cool like that.
Anyways, yeah, you should actually ignore me because I'm a college freshman who hasn't even ever taken any classes on anything related to human culture. Though I do think that my main point, that culture is a giant fluctuating web made of points which are individuals, is a good one.