Hooking up involves a sexual encounter

Hooking up involves a sexual encounter that involves people who are strangers or brief acquaintances, without an expectation of forming a committed relationship
Hooking up tend to be really popular on people within the age group of 15-28 because during these age group, the majority of people still haven't figured out what they want in life and do not want to take personal relationship too seriously.

This one hard but um i have no idea i’m sorry

An informal structure in a bureaucracy would be something like so-called "office politics". where there's a formal structure set up for the organization, but then an informal subculture as well that has a way of organizing the bureaucracy further. this can be positive because it sets up an added degree of organization, albeit informal, within the bureaucracy.