protected abstract void OnUpdate()

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 RequireForUpdate@@@@1 or ComponentSystemBase.RequireForUpdate in the system's OnCreate method. For more information, see ShouldRunSystem.

Use IJobEntity or SystemAPI.Query for common iteration patterns. You can also instantiate and schedule an IJobChunk instance; implement other Unity.Jobs interfaces or 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.