Singletons in Unity ECS - Code from tutorial video: https://youtu.be/oSj6w8AeeAg

ApplyRadiationSystem.cs

using Unity.Entities;

namespace TMG.ECS_Singletons
{
    public class ApplyRadiationSystem : SystemBase
    {
        private EndSimulationEntityCommandBufferSystem _endSimulationEntityCommandBufferSystem;

        protected override void OnCreate()
        {
            RequireSingletonForUpdate<RadiationTag>();
            _endSimulationEntityCommandBufferSystem = World.GetOrCreateSystem<EndSimulationEntityCommandBufferSystem>();
        }
        
        protected override void OnUpdate()
        {
            var ecb = _endSimulationEntityCommandBufferSystem.CreateCommandBuffer().AsParallelWriter();
            var deltaTime = Time.DeltaTime;
            Entities.ForEach((Entity e, int entityInQueryIndex, ref TimeToLiveData timeToLive) =>
            {
                timeToLive.Value -= deltaTime;

                if (timeToLive.Value <= 0)
                {
                    ecb.DestroyEntity(entityInQueryIndex, e);
                }
                
            }).ScheduleParallel();
            
            Dependency.Complete();
        }
    }
}

RadiationTag.cs

using Unity.Entities;

namespace TMG.ECS_Singletons
{
    [GenerateAuthoringComponent]
    public struct RadiationTag : IComponentData {}
}

SpawnCapsuleAuthoring.cs

using System.Collections.Generic;
using Unity.Entities;
using UnityEngine;
using Random = Unity.Mathematics.Random;

namespace TMG.ECS_Singletons
{
    public class SpawnCapsuleAuthoring : MonoBehaviour, IDeclareReferencedPrefabs, IConvertGameObjectToEntity
    {
        public float SpawnInterval;
        public GameObject CapsulePrefab;
        public Vector3 MinSpawnPosition;
        public Vector3 MaxSpawnPosition;
        public KeyCode FunKey;
        
        public void DeclareReferencedPrefabs(List<GameObject> referencedPrefabs)
        {
            referencedPrefabs.Add(CapsulePrefab);
        }

        public void Convert(Entity entity, EntityManager dstManager, GameObjectConversionSystem conversionSystem)
        {
            var spawnCapsuleData = new SpawnCapsuleData
            {
                SpawnInterval = SpawnInterval,
                SpawnTimer = SpawnInterval,
                EntityPrefab = conversionSystem.GetPrimaryEntity(CapsulePrefab),
                FunKey = FunKey,
                Random = Random.CreateFromIndex(0),
                MinSpawnPosition = MinSpawnPosition,
                MaxSpawnPosition = MaxSpawnPosition
            };
            dstManager.AddComponentData(entity, spawnCapsuleData);
        }
    }
}

SpawnCapsuleData.cs

using Unity.Entities;
using Unity.Mathematics;
using UnityEngine;
using Random = Unity.Mathematics.Random;

namespace TMG.ECS_Singletons
{
    public struct SpawnCapsuleData : IComponentData
    {
        public float SpawnInterval;
        public float SpawnTimer;
        public Entity EntityPrefab;
        public KeyCode FunKey;
        public float3 MinSpawnPosition;
        public float3 MaxSpawnPosition;
        public Random Random;
        public float3 RandomSpawnPos => Random.NextFloat3(MinSpawnPosition, MaxSpawnPosition);
    }
}

SpawnCapsuleSystem.cs

using Unity.Entities;
using Unity.Mathematics;
using Unity.Transforms;
using UnityEngine;
using Random = Unity.Mathematics.Random;

namespace TMG.ECS_Singletons
{
    public class SpawnCapsuleSystem : SystemBase
    {
        private SpawnCapsuleData _spawnCapsuleData;

        protected override void OnStartRunning()
        {
            _spawnCapsuleData = GetSingleton<SpawnCapsuleData>();
        }

        protected override void OnUpdate()
        {
            var spawnTimer = _spawnCapsuleData.SpawnTimer -= Time.DeltaTime;
            
            if (spawnTimer <= 0)
            {
                var newEntity = EntityManager.Instantiate(_spawnCapsuleData.EntityPrefab);
                
                var newTranslation = new Translation()
                {
                    Value = _spawnCapsuleData.RandomSpawnPos
                };
                
                EntityManager.SetComponentData(newEntity, newTranslation);
                _spawnCapsuleData.SpawnTimer = _spawnCapsuleData.SpawnInterval;
            }

            if (Input.GetKeyDown(_spawnCapsuleData.FunKey))
            {
                var newSpawnData = new SpawnCapsuleData()
                {
                    EntityPrefab = _spawnCapsuleData.EntityPrefab,
                    FunKey = _spawnCapsuleData.FunKey,
                    MinSpawnPosition = new float3(12.5f, 0.8f, 12.5f),
                    MaxSpawnPosition = new float3(12.5f, 0.8f, 12.5f),
                    Random = Random.CreateFromIndex(1),
                    SpawnInterval = 0.01f,
                    SpawnTimer = 0.01f,
                };
                
                SetSingleton(newSpawnData);
                _spawnCapsuleData = GetSingleton<SpawnCapsuleData>();
            }
        }
    }
}

TimeToLiveData.cs

using Unity.Entities;

namespace TMG.ECS_Singletons
{
    [GenerateAuthoringComponent]
    public struct TimeToLiveData : IComponentData
    {
        public float Value;
    }
}