What does Yunior say is the differences between the different types of girls?

A whitegirl will be brought by her parents because you live in the projects, which are unsafe. A girl from the projects will just "flow" in when she's ready. Blackgirls often want you to come meet their mom, which you do, running your hand through your hair like a whiteboy. Halfies arrive with their parents who insist the girl is white.