Cook the elbow macaroni in a large pot of boiling water until al dente, about 7–9 minutes.
Drain the macaroni thoroughly and rinse under cold water; let drain completely in a colander.
In a large bowl, whisk together mayonnaise, apple cider vinegar, granulated sugar, salt, and black pepper.
Stir in whole milk until the dressing is smooth and creamy.
Add finely grated carrot, finely diced celery, and finely minced yellow onion to the dressing and stir to combine.
Add the drained macaroni to the bowl and gently fold until every piece is coated in the dressing.
Taste and adjust seasoning with more salt or sugar if desired.
Cover the bowl and refrigerate for at least 1 hour (ideally 4 hours) before serving.