Originally, when Docker (the company) introduced Docker containers, everything was open source. Docker didn't have any commercial products at this time. The Docker engine that the company developed was a monolithic piece of software. It contained many logical parts, such as the container runtime, a network library, a RESTful (REST) API, a command-line interface, and much more.
Other vendors or projects such as Red Hat or Kubernetes were using the Docker engine in their own products, but most of the time, they were only using part of its functionality. For example, Kubernetes did not use the Docker network library for the Docker engine but provided its own way of networking. Red Hat, in turn, did not update the Docker engine frequently ...
