Cities

A writer of contemporary novels describes modern cities - London, New York, Stockholm, Amsterdam etc - that readers can check out for themselves.

A historical novelist describes, e.g., London, Rome or Paris as they would have been in earlier centuries.

A fantasy author describes imaginary cities inhabited by exotic beings like wizards, trolls etc.

A science fiction writer may describe any of the above - rationalizing the wizardry or trollery if wants to remain within the bounds of sf instead of diverging into fantasy. He may also describe:

Paris in an alternative timeline (see here);
cities on Earth in the future (see here and here);
human cities on other planets in the future (see here);
alien cities (see here).

As Brian Aldiss remarked once, an sf writer works hard for his living.