Isn't soccer a mostly women's sport in the USA? Do boys even play it at all?

The USA women's team is world class, probably for this reason.

In the UK boys mostly play soccer while girls traditionally played netball (basketball).

Soccer is extremely widespread and played by both boys and girls at a young age (up until puberty or so), but there definitely is a gender gap after that. I'd guess that there are a lot of other sports that are almost solely played by boys, so boys tend to drift away from soccer, while there are fewer options for girls. (Though there are some - lacrosse and softball for a couple examples.)