Cook the elbow macaroni in salted boiling water according to package instructions until al dente, about 8-10 minutes. Drain and rinse under cold water.

In a large mixing bowl, combine mayonnaise, milk, sugar, apple cider vinegar, salt, and pepper. Whisk until smooth.

Fold in shredded yellow onion and carrot into the dressing mixture.

Combine the cooled macaroni with the creamy dressing in a large bowl. Mix well until evenly coated.

Cover and refrigerate for at least 1 hour to allow flavors to meld. Stir in milk if needed before serving.