The branch of govt. that is mainly responsible for introducing, debating and passing laws is the

It is the legislative branch. It introduces the law(s) on a bill to Congress. But, during this process, anybody can right it. Next, it's introduced to an appropriate committee that consists of 17 senates, 70 subcommittees, 23 house committees, with 104 more subcommittees. The bill is considered by the subcommittees, then approved by the full committee, and goes onto the debate process. Making sure that the bill has went through all branches.

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.