Excellent window into this issue in the form of a podcast of This American Life episode 250, act 2, 'Detroit is in the House'. The show is pretty much 'a day in the life' of a member of the Michigan State House of Reps, but includes a backstory on how as a rooky, the member actually DID read every bill, which led to a surprising outcome...but with experience he no longer does so, trusting input from his fellow party members.
But apart from that show's portrayal (trigger warning: it presents a liberal-friendly version of events) of the legislative process, I'd say any complex piece of legislation is the result of a process like that which generates the complex machinery/operations such as a car or school: a car's many systems are designed by teams and then degree to which they work for the intended, overall result is given a thumbs up/down by higher-up executives, or in the case of a school, the complexity is added over time as schools have taken on more tasks and specialized staff were added, and layers of bureaucracy/procedures and so on. With a big, mega-bill piece of legislation, there are separate components that were worked on in committees, those pieces are linked together, but also, the complex tasks and issues dealt with by a mega-bill have developed over time with past examples guiding those involved in making this year's version.