One of the ways to think about the relationship between a made-up world and the real one is with the anthropological concept of a cultural sphere.
The easiest way for me to illustrate this is by pointing at the animated film Moana. You will not find the island of Motonui on any map; it isn’t Hawaii or Samoa or New Zealand. But it does feel like, if you were to sail around the tropical Pacific for long enough, you might find yourself on the shores of Motonui — and that’s because it was built to fit into the Polynesian cultural sphere. Which makes it different from, say, Dorne in George R.R. Martin’s series, where you might be able to trace similarities between certain aspects of life there and real-world societies, but there’s no single place where it very obviously belongs.
The concept of a cultural sphere is one with inherently fuzzy boundaries, because it’s about a cluster of traits which will be found in greater or lesser proportion in a given society. Often a cultural sphere is at least partly bounded by language: the Polynesian languages are related, and so are the Germanic ones, and the clusters of traits we call the Polynesian or Germanic cultural spheres tend to map to where those languages are traditionally spoken. This is because it’s easier to transmit cultural practices between groups who can communicate more easily — or, in the case of Polynesia, what happened is that a particular ethnic group spread out and settled a wide swath of islands, bringing their culture with them. Over the centuries those traits changed, with some of them vanishing and others evolving to be slightly different (the same way their languages evolved), until you wind up with a bunch of places that clearly resemble each other, without being quite the same.
But it doesn’t require a language family to happen. You can also have a sphere that’s created by the influence of some strong institution: both Catholicism and Islam have spread their particular stamp across large portions of the world. Obviously there’s still a linguistic element there — Latin for Catholicism; Arabic for Islam — but the cultural package has been adopted in part or in whole by regions where the indigenous languages are Mande, Austronesian, or Uto-Aztecan rather than Romance or Semitic.
The traits in question can be practically anything. Religion is usually an easy one to spot; the Catholic and Islamic spheres are both specifically religion-based, but elsewhere you can see things like the cognate relationship between Odin and Wotan, where differing mythologies closely resemble one another. The foods people eat; those are going to be shaped by the local environment, but when you get something like the Polynesians spreading out, they bring their staple crops and livestock with them, and an imperial society may do the same thing. Writing systems: the Sinosphere, the area of Chinese influence, can be mapped out in part by seeing where people adopted the Chinese writing system — even when, as in the case of Japanese, it made for a very poor fit.
Not everything in the sphere is big and load-bearing, though. It might be a particular game, like the ballgame common throughout pre-colonial Mesoamerica. (Though to be fair, that one had enough religious and political significance that maybe it shouldn’t be considered small.) It might be a particular style of artwork. It might be a folktale rather than a sacred myth, a story told just for entertainment, but found in a number of related forms throughout that region. But the key thing is that it’s never just one of those things. It’s a whole cluster of them, and while any given society within that sphere may not have all the traits, their Venn diagrams overlap enough to be meaningful.
What’s the significance for this and writing? To begin with, when people talk about “Eurofantasy,” there’s a sense in which that’s a statement about a cultural sphere. Or perhaps several, since there are distinct strands of Germanic, Celtic, and post-Roman Eurofantasy out there, which loosely replicate the traits of Norse or Irish or medieval European society. Many of the writers of such books don’t even realize they’re doing that, so being aware of societies as packages of interlinked traits can help you be more alert to which packages you’re drawing your ideas from.
It also describes a distinct space which I’ve taken to calling “world-and-a-half,” as a facetious way of pointing out that while it’s technically a secondary world, it isn’t fully divorced from the primary one. The map may be different, the history may hit different beats, but the cultures are recognizable. This is the kind of thing Guy Gavriel Kay regularly writes, though he’s far from the only one. It’s the space you head for when you want something that has the feel of India or Egypt or the Pacific Northwest, without locking yourself into the exact historical facts of a particular point in time (or spinning out a specific alternate history). By familiarizing yourself with the traits of that sphere, the shared commonalities between its constituent societies — and also their differences! — you can make better decisions about how to craft your society to evoke that feel, while still maintaining the freedom to move around in your fictional world.
And when you’re not trying to fit into a cultural sphere, but rather to make something less singular in its inspiration, being aware of this concept can help you avoid some pitfalls. As I’ve said, these traits are interlinked. Not rigidly; if they were, then you wouldn’t get the variations within a sphere that happen in the real world. But it’s still true that you can’t necessarily yank X out entirely and still have Y make sense. Medieval European kingship was heavily rooted in Catholicism, from its ritual to its politics to the underlying cosmological idea of what a king should be; if you de-Catholicize your medieval fantasy and replace it with something like Shinto, but don’t rework the monarchy to account for that, you may leave your reader with a niggling feeling that this doesn’t really hang together. Some instances of cultural appropriation amount to a failure to notice the interconnection between cultural traits, such that the author grabbed the shiny obvious one and ripped it away from its necessary underpinnings.
It can be difficult to see how things are connected, though. Cultural spheres don’t come with instruction manuals telling you how they’re assembled. Like everything else in worldbuilding, it helps to read widely — not just in fiction, but in nonfiction that discusses real societies and how they work. It’s more an osmotic process than a step-by-step one. But having researched Polynesian cultures to make my own fictional Polynesian society for Voyage of the Basilisk, I was delighted to see those familiar traits crop up in Moana, and the same goes for any other story that triggers that spark of recognition. I do love a good, original-feeling secondary world — but there’s something about a world-and-a-half that makes me smile.