World concepts
A world is a collection of entities. An entity's ID number is only unique within its own world. A world has an EntityManager struct, which you use to create, destroy, and modify the entities within the world.
A world owns a set of systems, which usually only accesses the entities within that same world. Additionally, a set of entities within a world which have the same set of component types are stored together in an archetype, which determines how the components in your program are organized in memory.
Initialization
By default, when you enter Play mode, Unity creates a World instance and adds every system to this default world.
If you prefer to add systems to the default world manually, create a single class implementing the ICustomBootstrap interface.
If you want full manual control of bootstrapping, use these defines to disable the default world creation:
#UNITY_DISABLE_AUTOMATIC_SYSTEM_BOOTSTRAP_RUNTIME_WORLD: Disables generation of the default runtime world.#UNITY_DISABLE_AUTOMATIC_SYSTEM_BOOTSTRAP_EDITOR_WORLD: Disables generation of the default Editor world.#UNITY_DISABLE_AUTOMATIC_SYSTEM_BOOTSTRAP: Disables generation of both default worlds.
Your code is then responsible for creating your worlds and systems, plus inserting updates of your worlds into the Unity scriptable PlayerLoop.
Unity uses WorldFlags to create specialized worlds in the Editor.