I created this video guide to help explain Unity ECS

Unity Engine
,
4

The automatic creation is done in AutomaticWorldBootstrap.cs in the com.unity.entities package.

Basically the file looks like this

using UnityEngine;

namespace Unity.Entities
{
#if !UNITY_DISABLE_AUTOMATIC_SYSTEM_BOOTSTRAP
    static class AutomaticWorldBootstrap
    {
        [RuntimeInitializeOnLoadMethod(RuntimeInitializeLoadType.BeforeSceneLoad)]
        static void Initialize()
        {
            DefaultWorldInitialization.Initialize("Default World", false);
        }
    }
#endif
}

So to disable automatic creation just make sure to set the define UNITY_DISABLE_AUTOMATIC_SYSTEM_BOOTSTRAP.

Then all you need to do is create your world (line 30 below) and make sure Unity knows about it (line 59 below), and dont forget to make sure you also clean up on DomainReload (line 38 below). To see how this is done check DefaultWorldInitialization.cs below

using System;
using System.Linq;
using UnityEngine;

namespace Unity.Entities
{
    static class DefaultWorldInitialization
    {
        static void DomainUnloadShutdown()
        {
            World.DisposeAllWorlds();
            ScriptBehaviourUpdateOrder.UpdatePlayerLoop();
        }

        static void GetBehaviourManagerAndLogException(World world, Type type)
        {
            try
            {
                world.GetOrCreateManager(type);
            }
            catch (Exception e)
            {
                Debug.LogException(e);
            }
        }

        public static void Initialize(string worldName, bool editorWorld)
        {
            var world = new World(worldName);
            World.Active = world;

            // Register hybrid injection hooks
            InjectionHookSupport.RegisterHook(new GameObjectArrayInjectionHook());
            InjectionHookSupport.RegisterHook(new TransformAccessArrayInjectionHook());
            InjectionHookSupport.RegisterHook(new ComponentArrayInjectionHook());

            PlayerLoopManager.RegisterDomainUnload (DomainUnloadShutdown, 10000);

            foreach (var ass in AppDomain.CurrentDomain.GetAssemblies())
            {
                var allTypes = ass.GetTypes();

                // Create all ComponentSystem
                var systemTypes = allTypes.Where(t =>
                    t.IsSubclassOf(typeof(ComponentSystemBase)) &&
                    !t.IsAbstract &&
                    !t.ContainsGenericParameters &&
                    t.GetCustomAttributes(typeof(DisableAutoCreationAttribute), true).Length == 0);
                foreach (var type in systemTypes)
                {
                    if (editorWorld && type.GetCustomAttributes(typeof(ExecuteInEditMode), true).Length == 0)
                        continue;
                      
                    GetBehaviourManagerAndLogException(world, type);
                }
            }

            ScriptBehaviourUpdateOrder.UpdatePlayerLoop(world);
        }
    }
}
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