Staffers. No way one person can do the entire review and not miss something. And you know the CONgress critters are too important to do it themselves. Thus, staffers. Who may have their own agendas and fail to mention certain things? A mess? Yup.
I would assume lobbyists write the parts to support their agenda and this is put together in one bill. Does anyone thing Nancy wrote this while on a break in California?