This depends on the manufacturer. Today, many kids bikes are unisex, but those designed specifically for girls tend to have a step-through frame - traditionally used to accommodate skirts. Today, it just makes the bike a little easier to mount, which could be a boon for many children regardless of gender.
For kids, purchasing a gender specific bike isn’t necessary - they haven’t yet developed to the point where their bodies are different enough to warrant special modifications. A few companies will still insist on selling pretty pink bikes for girls, and rugged blue bikes for boys, but there’s no physical reason to choose one over the other - it all boils down to your child’s personal preference.
In adulthood, things are a little more complicated. Because women tend to be shorter and lighter than men, bikes designed specifically for women often feature a shorter stack and reach length, for a more comfortable riding position. Many women find a better fit with unisex or men’s bikes, though - it all depends on an individual’s build.