Kubernetes uses the Docker daemon as its container runtime. The docker daemon is running on all your hosts, and Kubernetes talks to the daemon api to start up containers.
user_request -> kubernetes api -> docker api on a host
You can drop in other runtimes, like using containerd directly.
You should check out what docker and kubernetes actually are if you'd like to discuss them. Kubernetes is a management engine that controls docker containers, replacing the docker developed solution.