Comment by nonameiguess

You probably need to change the thought experiment to get at what you're really trying to get at. If all man-made things disappeared tomorrow, you're talking pipes, wires, streets, houses, antibiotics, the immediate effect is very close to 8 billion people die within a few weeks. The hundred thousandor so survivors may know what they already knew, but now they need to find a way to find each other, write it all down, without pens or paper, make more people, and teach those new people so the knowledge isn't lost within a generation or two, all while also building enough infrastructure to at least keep the tiny communities still around from also starving.

All of the post-apocalypse LARP scenarios other responses are describing tend to assume at least books and some small number of durable structures survive. If nothing at all makes it, the first thing that happens is most everyone not living on a ground floor dies instantly from falling. Everyone living where it freezes dies in a few hours from exposure. Most people living anywhere experiencing a heat wave die from heat exhaustion in the next day or two. Most everyone else everywhere else dies from starvation or disease in the next few weeks. Even if you're living on a farm, you've got no fences or tools, so you better hope it's a farm with lots of edible plants and it's near harvest season. If you live in a city, you have no hope at all. Even existing food stores will spoil quickly with no containers or refrigeration. The people who do the best will be Hadza and uncontacted tribes and what not that lose their spears and tents, but all they have to do to get back to their prior situation is make more spears and tents, which thankfully doesn't require a global transportation network or modern manufacturing techniques. They're not the ones who know how to build iPhones, though. I'm not sure anyone knows how to build an iPhone, really. The knowledge is probably largely digital. The chief engineer at Apple knows where to look and who to ask, but no single person has all of the knowledge in their own head.