Kubernetes is pretty simple, compared to some of those. The "master" servers read from Etcd and talk to daemons on the worker servers (Kubelet) to start/stop Docker containers, or to cloud APIs to manipulate volumes and network configuration. Then there's just a web server to answer API requests to read/write Etcd.

There are some components running on top of this to provide networking, DNS, logging, etc, which simply run as containers on all machines.

Of course there are many types of objects, many possible addons for logging/network/..., and the API authentication is quite complex, but the overall concept is quite simple and builds directly on Etcd.