1. The general consensus is that there were more people. This is assumed in Genesis and it (annoyingly!) doesn't bother to explain it, as the audience at the time already assumed it. Also, the authors weren't interested in all the logistics and technicalities that we are today.
2. Cities referenced in Genesis were likely fortified settlements, rather than like modern cities.
The idea that people in Africa could only build simple huts is a myth that came from the colonial era. Africa had large cities, architecture and metallurgy while parts of Europe were still tribal.
If you're keen to learn more, there are some good books that explain this much better than a comment can, such as "How to Read the Bible for All Its Worth" by Fee & Stuart and "Genesis for Normal People" by Pete Enns. I haven't read it but "African Civilizations" by Graham Connah is probably the go-to book on how African cities and technologies were so much further ahead than traditional European/US narratives place them.
The best resource for these kinds of questions is probably "The Bible Project". They have a load of YouTube videos and podcasts that cover these kinds of questions.