The difference between buildings and settlements is one of scale. As Aldo Van Eyck once said, "A building is a small city; a city is a large buiding."
I would be grateful if you would tell me what the first sentence means. Is it a synonym for city? Thanks!
One definition of a "settlement" is a place where people come to live. It could be a colony from elsewhere, or people who have journeyed into a frontier. The group of buildings/homes/etc that they build is called a "settlement."
What he is saying is that if one person puts up a few buildings, it is not a settlement. You need a certain number of people to make a settlement.