Entity is an ID. You can think of it as a super lightweight GameObject that does not even have a name by default.
You can add and remove components from entities at runtime. entity ID's are stable. They are the only stable way to store a reference to another component or entity.
You can add and remove components from entities at runtime in much the same way as a
GameObject. Entities can be created from Prefabs by using
EntityManager will parse the Prefab for ComponentData and add it when it creates the entity.
Iterating over all entities that have a matching set of components, is at the center of the ECS architecture.