There are many different subgenres of fantasy fiction, but in my experience, one of the major divisions is whether the stories take place in something that is recognizably our own world, or a secondary (made up) world. Real-world fantasy can take place in contemporary settings (as do most works of urban fantasy), historic settings or mythic settings (like Arthurian Britain or Olympian Greece), and of course different writers embrace these settings with different degrees of historic rigor or realism. There are also portal worlds, which are secondary but connected to our own in some way (as the Chronicles of Narnia) and "Wainscot" worlds, where a hidden magical society with its own rules is tucked away, hidden from the perceptions of the non magical (The Harry Potter books and Neil Gaiman's Neverwhere are examples of this).
But for many readers, the epitome of fantasy lies in secondary worlds that are not connected to our own. In these tales, the protagonists are born, live and die with no knowledge of the Earth and its history or gods. When I decided I wanted to get serious about fantasy writing a few years ago, it never occurred to me to write anything else. Many of my favorite fantasy novels were of this type, and the concept of making up my own world, with its own history, culture and rules was too exciting to pass up.
My current projects take place in a secondary world. The task creating a fleshed-out and internally consistent world with cultures, geography, religions, government, magic systems, ecology and technology that don't feel like they were drawn randomly from an epic fantasy hat is harder than it looks, however. As I try to polish my first novel for submission and smooth out the rough edges, I'm having a lot of those "if I only knew such and such when I started" moments.
One thing I've been thinking about lately is where my world actually came from. I'm not a fan of the old convention of writing a long, info-dumpy prologue that details the origins and history of my world. I also don't like those long "omniscient" outtakes that some writers insert into narrative. My story is written in limited third, and I'm being strict with it in that I'm not telling or showing the reader anything my pov characters don't know or wouldn't think.
But this doesn't mean that I shouldn't know why things are the way they are. Even if my characters have no idea about their world's true history, or even have wrong assumptions about it, the nature of their world will shape their views and perceptions in some way. And there will be little hints about the origins and history of their world in various aspects of its biology and geography.
I've been thinking about the different ways a world can (in a fantasy sense) come to exist, and I've come up with three broad categories.
1. A naturalistic or evolved world like our own. Such a world (if reasonably Earthlike) would be billions of years old and would have a history of complex life that goes back hundreds of millions of years. There may be magic and gods and fantastic creatures or beings, but they're aspects of this natural world. If a god or gods created it, they'd be like the deist's god--setting things up at the beginning and letting them run according to the rules. Perhaps the gods even evolve and change with the world. The people who live in the world may well be unaware of its true nature (just as we were unaware of the true nature of our own for much of its history, and indeed, many still are). They may believe in gods that don't exist, or completely misunderstand the god or gods that do exist.
There are certain things that one expects to see in an evolved world: fossils and fossil fuels (like coal), homologies between the body parts of different types of organism, sensible biogeographical distribution of plants and animals, ethnicities and cultures that fit in with what we understand about the effects of biogeography on human micro-evolution. Of course, if your world isn't very much like Earth, things can get interesting here. But in general, the author might approach things more the way a science fiction writer might, though the presence of magic and interactive gods does make it possible for some bizarre incongruities to exist (say dragons exist and were created by a long-ago wizard as a bioweapon).
2. A created world. This is a world where the reality is as many myths and legends from our own history have asserted--there is a god or gods who made the world, probably just a few thousand years ago, though it could be older, and it hasn't changed much since (barring god-created or magical cataclysms). The world doesn't "have" to be made by gods, of course. It could be the creation of technology or magic wielded by mortals. But it's implicit somehow that there really is a creator and that humans (and other intelligent and non-intelligent species) have been around in pretty much their present form all along.
While the people in an evolved world might believe they live in a created world, there will be some things one with a more scientific mind set might expect to see in a created world that would not be present in a naturalistic one. Most notably, there would be matters of geography, species distribution and homologies between different kinds of creatures (fantastic and otherwise). In a created world, there's no reason why fantastic creatures would need to have internal structures that made it clear they evolved from the same ancestors as other animals with backbones. There is also no need for there to be fossils, there likely wouldn't be coal or petroleum (although the gods might have something analogous that they gifted people with) and so on.
3. An invaded world. In essence a portal world where someone or something came from somewhere else in the recent or distant past and either mostly supplanted the original denizens of either an evolved or created world, or are a thorn in the side of said denizens. This can be the "real" reason there are mixtures of fantastic and mundane creatures and beings where some seem to follow predicable evolutionary patterns and others don't. This is a portal world, maybe, where the portal is now closed or vanished.
I think most traditional fantasy falls into the second category. A god or gods *really* did create the world, and this reality is reflected in the beliefs of at least one group of people on the world (though possible through a glass, darkly). Tolkien's and Lewis's worlds are certainly like this, and it was strongly implied that Earthsea was (Ged knew that Segoy raised the islands from the sea and created the dragons first, though we never learn how Ged knew this).
Often it's not completely clear which of these models the author had in mind, unless some unambiguous "truth" is revealed to the characters. In fantasy, it's not necessary to really understand all the whys and wherefores. Indeed, trying to drop them into a story can be klunky or heavy handed. As a reader, I don't know why George RR Martin's world, for instance, has its strange seasonal cycle (though I've wondered if it's because it has a really bizarre axial wobble paired with a very active and varied sunspot cycle). I don't really need to know why to enjoy the story.
However, if you know the reason for things being the way they are, you can incorporate this knowledge in ways that will make your world building more internally consistent. Even if my readers (and characters) never know for sure where their world came from, I do.
Happy Longest Night to everyone, and I hope everyone has a wonderful Christmas (or Yuletide, or Solstice, or winter holiday).