“Kubernetes is a portable, extensible, open source platform for managing containerized workloads and services, that facilitates both declarative configuration and automation.”
~ https://kubernetes.io/docs/concepts/overview/what-is-kubernetes/ ~ https://www.spectrocloud.com/blog/why-kubernetes-now-a-beginners-guide-to-modern-cloud-native-infrastructure/
In other words, Kubernetes is an API for managing application containers in a declarative way. It is open and flexible. Core components are provided by the platform, the rest is on you.