Building the world where magic is real
The contemporary fantasy world is built by asking, systematically, what the contemporary world would actually look like if magic were present: not just what individual characters can do, but what institutions exist, what economic structures have developed, what social norms govern magical ability, what the relationship between magical and non-magical communities looks like, what the legal framework is. Building this world requires following the logic of magic's presence through the world's systems rather than stopping at what is immediately relevant to the plot. The world that has been thought through rather than only the plot-relevant parts of it is the world the reader will believe in, even if much of what was thought through never makes it onto the page.