Using Jobs to Edit Unity PhysicsVelocity on Entities

I'm trying to add some velocity to physics entities once their velocity drops below a certain threshold. I'm having some trouble understanding the Jobs System and how it relates to ECS and Entity Components. From what little information I can find, getting Entity Components in a Job requires some steps that don't have many examples.

Here is how I'm scheduling the Job:

Code (CSharp):

1.         PhysicsJob.AddRandomPhysicsJob job = new PhysicsJob.AddRandomPhysicsJob()
2.         {
3.             entityArray = test,
4.             velocities = foundVelocities,
5.             minVel = m_MinVel,
6.             randomXPos = randomX,
7.             randomYPos = randomY,
8.             randomZPos = randomZ,
9.         };
10.  
11.         JobHandle jobHandle = job.Schedule(entityArray.Length, 32);
12.         jobHandle.Complete();

And here is how the job is structured:

Code (CSharp):

1.  
2. public class PhysicsJob : JobComponentSystem
3. {
4.     private EndSimulationEntityCommandBufferSystem endSimulationEntityCommandBufferSystem;
5.     protected override void OnCreate()
6.     {
7.         endSimulationEntityCommandBufferSystem = World.GetOrCreateSystem<EndSimulationEntityCommandBufferSystem>();
8.         base.OnCreate();
9.     }
10.     public struct AddRandomPhysicsJob : IJobParallelFor
11.     {
12.         public NativeArray<Entity> entityArray;
13.         public NativeArray<Unity.Mathematics.float3> velocities;
14.         public float minVel;
15.         public float randomXPos;
16.         public float randomYPos;
17.         public float randomZPos;
18.         public EntityCommandBuffer.Concurrent entityCommandBuffer;
19.         public void Execute(int index)
20.         {
21.             Entity entity = entityArray[index];
22.             Unity.Mathematics.float3 vel = velocities[index];
23.             Vector3 tempVel = new Vector3(vel.x, vel.y, vel.z);
24.             if ((tempVel.magnitude < minVel))
25.             {
26.                 entityCommandBuffer.SetComponent(index, entity, new PhysicsVelocity() { Linear = new Unity.Mathematics.float3(randomXPos, randomYPos, randomZPos) });
27.                 entityCommandBuffer.SetComponent(index, entity, new PhysicsDamping() { Linear = 0.01f, Angular = 0.05f });
28.             }
29.         }
30.     }
31.     protected override JobHandle OnUpdate(JobHandle inputDeps)
32.     {
33.         AddRandomPhysicsJob job = new AddRandomPhysicsJob()
34.         {
35.             entityCommandBuffer = endSimulationEntityCommandBufferSystem.CreateCommandBuffer().ToConcurrent()
36.         };
37.         endSimulationEntityCommandBufferSystem.AddJobHandleForProducer(inputDeps);
38.         return inputDeps;
39.     }
40. }
41.  
42.  

Currently, my understanding is that I construct the job and pass it the needed info with the "new" keyword.

However, I'm getting confused with the difference between defining an IJobparallelFor struct in a monobehavior, and sticking it in a class that inherits from JobComponentSystem. What's the difference? Is this even the right approach for checking the component velocities and adding a random velocity?