We call it world-building; no unified field-theory covers all observed data.
Most models require one Grand Map pairing with one Encyclopedia; these collide to spin off races and faces, languages, currencies, histories and customs down to table placement for royal dinners. Success by this model of world building is measured by (quantity of books times quantity of characters) to the nth story-arc squared. A clear and fair formula.
But sometimes world-building is something different. It is the evocation of another reality. Soon or late, this feeling comes to most lovers of fantasy. Not often enough; but it drives them to wander story-wastelands, seeking the italicized thrill again.
The sensation is evoked by descriptions of scenes, conversations, ways of thinking and acting by characters who seem strange, absurd, impossible; and yet immediately recognizable. Like music you never heard before, yet you know the coming notes.
Eric Rücker Eddison (1882-1945) was a British civil servant when the sun dared not set upon an ordered Empire, and servants knew how to set a proper tea. A scholar of Old Norse, he wrote one fantasy book of note. It is a world in a book. Not an easy read. Not a believable world. Yet it has evoked that ‘other-place’ feel in so many notable authors, it ranks as a major inspiration to fantasy's desire to give readers that shiver again.