Bring a large pot of generously salted water to a boil.
Cook the macaroni until very soft, about 2 to 3 minutes beyond al dente.
Drain immediately and transfer to a large mixing bowl.
While still hot, stir in the apple cider vinegar and ¾ cup of the milk.
Let the macaroni cool completely, stirring occasionally as it absorbs the liquid.
In a separate bowl, whisk together the mayonnaise and remaining ½ cup milk until smooth.
Stir in the grated carrot, grated onion, salt, and pepper.
Add the dressing to the cooled macaroni and stir until evenly coated.
Cover and refrigerate for at least 4 hours, preferably overnight.
Stir before serving. If needed, add a splash of milk to loosen the dressing.