So how many people are capable of building a transistor? Where will those computer things come from after a couple of generations?
You might be surprised. Nothing has to be reinvented from scratch. The earliest versions of modern chip-making processes are now public domain, as the patents have expired. The patents themselves are less than useful as descriptions of what to actually do to make a chip, but they are legitimately a place to start for anybody bootstrapping a foundry. Those patents are old enough that many copies of them exist in printed form, so having a functional computer connected to a functional Internet is not a prerequisite for access.
But I don't see how it's possible to collapse that far any longer. Maybe before the ascendance of ARM chips, you had a good point. There were only two companies in the world making fully capable CPUs (I discount microcontrollers here because they're usually too specialized). Nowadays there's an ARM foundry around every corner. ARM cores may not be cutting edge in performance, but they can do all the scalar operations you need and enough of the vector operations. And they're dirt cheap and a lot of people all around the world know how to make them. The process is cheap and easy and can work in suprisingly primitive conditions. (Where "primitive" is a relative term, of course.) More to the point, there are ARM systems all over the world now. We're not quite to the point where there's a functional ARM system for every human on Earth, but we're very rapidly approaching that point. That kind of ubiquity is hard to lose once it's established. It would take a world wide religious pogrom to do away with sufficient numbers of those pocket computers to actually put a dent in their availability.
And as long as we have those pocket computers, we can hold things together. We have functional processors and data storage so vast that somebody, somewhere, has access to anything you need to know to keep civilization running, right down to how to produce ad hoc power solutions to keep it all working. Making a solar panel in your garage isn't really feasible, but making a multi-kilowatt wind turbine is astonishingly easy, especially when salvaging parts, and you could store complete plans for how to do so in a tiny fraction of your phone's storage device, available for the rest of your life.
Civilization is a lot more robust than many people imagine. Some of that robustness happens specifically because people imagine it isn't, and so they take steps to improve an already remarkably resilient system. If it bothers you, join the crowd. Storage for detailed plans and procedures for making every kind of machine required for at least a modicum of civilization costs less than $100, with room not just for blueprints, but for How To instructional videos for every piece of it. Leave out the video and depend on just detailed textual instructions and that storage can be solid state for the same price.