Boys are from other departments and girls are from the finance department. What does it mean to let that boy come here as her brother? What psychology?

This kind of joke may not have too deep psychological meaning, but it is just a way to adjust the working atmosphere and relieve the work pressure. In the workplace, colleagues often joke to enhance communication and familiarity. Therefore, boys don't need to pay too much attention to this joke, and face the way of getting along with colleagues and joking with a relaxed attitude. Girls may play this joke out of ridicule, or they may be eager to have a work partner who can trust and help each other at work.