> absent an authority to gather together all the experts with their tightly focused specializations, absent the chip fabs and the global supply network, how exactly do you build a computer? Or even something as “simple” as a pencil?

You develop that first. If there is a nuclear war and somehow I survive with 5000 people near me survive (if we are scattered around the world that doesn't help) that is enough for genetic diversity and we can start rebuilding society. There are plenty of books in libraries. I can figure out how to make a primitive printing press (likely from clay - fragile but just the ability to print say 10 books from a plate is huge and seems possible). Books in libraries will last for a while, and we can figure out better ways to store them. In many be 1000 years, but what of physics and chemistry we are able to preserve will help whoever. They will develop different supply chains. They will start knowing a spinning wheel is possible (I don't know enough about the spinning jenny to automate it) - I know where I live warm clothing is critical and so I will be sure to take the time to develop a spinning wheel thus allowing women to not have to spend 12 hours a day with a drop spindle. That is time they can then spending on investing something else while the men farm. (there is good reason for this sexual division of labor so I'd be forced to bring it back in the early days)