A fantasy world is a promise to the reader that it holds together. We start by asking what the world is for. Is it a stage for a war, a mirror for a modern problem or a place the heroine would die to protect? The purpose decides which details deserve pages and which can stay offstage.
Magic comes next. Some authors want a hard system with clear rules, limits and costs. Others prefer a softer sense of wonder where magic is mysterious and dangerous. Both work. What does not work is magic that solves problems the author did not want to write. We define what magic can do, what it cannot, who has it and what using it takes from the user.
Then geography, politics and culture. Who rules, who resents it, what do they worship, and how do they greet each other? Names are a good test. If every name sounds like the same invented language, readers lose track, and if none do, the place feels borrowed. We build a naming scheme per culture and keep a glossary.
All of this goes into a world bible that you approve. It is the single most useful document for a long project, because it lets a writer, an editor and a future cover designer see the same world.
Not all of it goes on the page. Readers should sense depth through a few well-chosen details, not through chapters of exposition.