Some is innate - I have 2, different types of intelligence, but both bright (one extremely so), but one of the biggest things is they are both driven, competitive, love learning. DH and I are similar. Hard to know what's genetic and what is due to how we've raised them due to being high performers ourselves.
DD isn't curious though. She works hard, is conscientious and has natural talents, BUT she has little curiosity about the world. DS OTOH is like a walking wikipedia, in addition to DD's habits/skills, he reads and reads and questions and researches and I am fairly certain he will pull far ahead as a result.
How do we support? Encouragement and praise for effort, provide lots of books, help them pursue their interests by researching things with them, find them suitable books to extend their learning, visit museums etc.
But one of the biggest things besides reading is the discussions we have. If they ask a question then they get a proper response, and if we don't know the answer we research with them. Discussions at home and out and about can be really deep. I don't treat them as children, but mini-adults deserving of proper, considered answers and discussion. I don't brush off a question with a quick answer, or platitude. If I can't discuss there and then we pick it up at dinner or something.