Entity component system introduction
The Entities package uses the entity component system (ECS) architecture to organize code and data for some of its processes. An entity is a unique identifier, like a lightweight unmanaged alternative to a GameObject. An entity acts as an ID associated with individual components that contain data about the entity. Unlike types that inherit from MonoBehaviour, entities contain no code, or data: they're a reference to units of data that the systems you create process.
The following diagram illustrates how entities, components, and systems work together:
In this diagram, a system reads Speed and Direction components, multiplies them and then updates the corresponding Position components.
The fact that entities A and B have a Renderer component but entity C doesn't makes no difference to the system, because the system has no knowledge of Renderer components.
You can set up a system so that it requires a Renderer component, in which case, the system ignores the components of entity C. Alternatively, you can set up a system to exclude entities with Renderer components, which then ignores the components of entities A and B.
The diagram also illustrates the concept of an archetype, which is a unique combination of component types.