How DNS carry out scalability or how exactly is zone information stored?

DNS is scalable in two respects, first, it's scalable in that it's able to store an increasing and ever-changing set of nodes, and second, it's scalable in the sense that it's not necessarily controlled by a single person or group. That way the management overhead of a very large namespace doesn't have to fall on a single entity which is achieved by dividing up the namespace. First, portions of the namespace, that is to say, domain names, are delegated to different people or groups so that they can modify only their portion and nobody else's. It's then the responsibility of those people to provide name servers to hold their delegated portion of the namespace. The net effect is that the entire namespace is distributed across different name servers such that no single server has to store the entire tree, each server stores only a subset of the tree.