ECS has three deployment controllers. The default ECS controller performs Rolling Update. The CODE_DEPLOY controller performs Blue/Green. EXTERNAL is controlled by external systems like App Mesh or Argo Rollouts. On the exam, "ECS Blue/Green" always means the CODE_DEPLOY controller. Get this distinction wrong and all subsequent answers are wrong.
The physical implementation of ECS Blue/Green is "two Target Groups + ALB Listener." Current Task Set registers with Blue Target Group, new Task Set with Green Target Group. Traffic switches via ALB Listener weight changes