We know from the show that tens of millions possible hundreds of millions survived given the population of the world at the time. We don't know if any these people got together for formed cities, but it's likely at some point. All the technology and knowledge isn't lost, so pretty much anything within reason can be made just as it was before just on a smaller scale and more intensive demanding. Possibly pockets of civilisation don't know about other pockets, though In my mind I imagine towns and cities and settlements, and trading. Also since crossing oceans is possible news can spread. The story we see seems to be centred around a group that shunned tech and innovations from the past (for the most part) and choose to live a simple life, but presumably that isn't the world at large.