void OnUpdate(ref SystemState state)

SystemState

By default, the system invokes once every frame on the main thread. To skip OnUpdate if all of the system's EntityQueries are empty, use the RequireMatchingQueriesForUpdateAttribute. To limit when OnUpdate is invoked, you can specify components that must exist, or queries that match specific Entities. To do this, call SystemState.RequireForUpdate or SystemState.RequireForUpdate in the system's OnCreate method. For more information, see SystemState.ShouldRunSystem.

You can instantiate and schedule an IJobChunk instance; you can use the C# Job System or you can perform work on the main thread. If you call EntityManager methods that perform structural changes on the main thread, be sure to arrange the system order to minimize the performance impact of the resulting sync points.