How could a girl move on so quickly after breakup than a guy ??

Depends on circumstances. Who initiated the breakup? Usually the imitator of a breakup has already been processing their feelings of the relationship ending before the end ever came.

There's also people that tend to be codependent and need someone so they move on quickly.

There's also people who use someone as a rebound outlet.

There's really add many different reasons as there are people.

Or sounds like you maintain a lot of girl vs guy stereotypes. If you want to understand people, you will have to set those aside.