Any sufficiently advanced technology is indistinguishable from magic.
Clarke’s third law.

The universe is full of forces; some of those forces are personalities.
reference lost

I believe that the second quotation above is from a piece by James Blish, which I read many years ago and no longer have available to me. If anyone recognizes the source, let me know. In that same piece, I believe, he spoke of Black Easter as an experiment in which he treated the Book of Revelation as simple fact. Roger Zelazny made a career out of treating non-Western religions as if they were simple fact.

Like stardrives, magic can be highly structured or haphazardly thrown in when the story needs it. Both styles work, depending on the skill of the author. The most organized magic I recall is Randall Garrett’s Lord Darcy stories. For disorganization, see any new author.

A primary, underlying question in any presentation of a fantasy world is, “Where does the magic come from?” Is it a dispersed, readily available natural resource like The Force in Star Wars? Does it reside within its wielder, and a natural consequence of his being? Or is it owned by other powerful beings, who must be supplicated or bargained with to obtain a portion of their power? This choice has a huge effect on how dark the story is likely to become.

Christianity, in earlier centuries, saw witches as wielders of power which they obtained by pacts with Satan. Harmful as they were, they had no power of their own. In many dark fantasies, the searcher after power obtains his heart’s desire from some greater being who is, in essence, a Satanic stand-in. Such Faustian bargains never end well.

Magic, in fantasy writing, often goes unexplained. The talisman in The Monkey’s Paw is understood by the reader without elaboration, just as a reader of westerns doesn’t need an explanation of how a six gun works.

It is quite usual for a fantasy hero to have inborn power. Harry Potter was a wizard born of wizards. Ged is an unknown until his power is discovered by a mage. Corwin is a son of Amber.

It is equally usual to concentrate on the education of a wizard, or mage, or dziai. Ged went to Roke; Harry Potter went to Hogwarts, and my Tidac took two books to learn how to use his power because he had no mentor. His father never learned, and it destroyed him.

Can we have fantasy without magic? Pavane is an alternate universe science fiction or an alternate history novel, but its tone makes it read like fantasy, except for the absence of magic. What seems to be magic in one chapter, may just be a dying dream; it isn’t made clear to the reader. For me, this places Pavane on the borderline between genres.

On the other hand, Randall Garrett’s Lord Darcy stories are all about magic, but their system of magic is so comprehensively worked out that they read like science fiction.

I know that my Menhir story, in its infancy, read like a quasi-medieval world. Slowly I came to grips with how the powers of every soul are affixed to menhirs at death, making menhirs into gestalt sentient beings which become repositories of power that can be tapped, at peril, by men of power. Only then did magic come into the world of the menhir. And only then did it begin to read as fantasy.