You find movies about the Civil War. You go to the zoo or a pet shop or a state fair to see chinchillas and talk to people who raise them. You find a TV program about tying flies and how to cast, and you go to the lake and see people fishing, and talk to them. You realize you can't quite understand how to do it by reading, so you find someone who can show you. You want to have some fun interaction with others, so you join a Civil War reanactment society.
People learn because they are interested in learning something, for some reason.
Children will learn long division, and algebra, and calculus in the same way. If they truly are not interested in mathematics, then they don't need it. They will most likely not pursue careers that require it. Basic arithmetic, sure. People need that, and without the interference of school, kids find it fun.