Bring a large pot of water to a boil, add the kosher salt, then stir in the elbow macaroni and cook until very soft and slightly overcooked, about 10–12 minutes.

Drain the cooked macaroni in a colander, do not rinse, and let it steam off for 5 minutes until most surface moisture is gone but the pasta is still warm.

In a small mixing bowl, whisk together the mayonnaise, whole milk, apple cider vinegar, granulated sugar, kosher salt, black pepper, onion powder, and garlic powder until smooth.

Transfer the warm macaroni to a large mixing bowl and pour about two-thirds of the dressing over it, stirring gently to coat every piece.

Fold in the shredded carrot, minced onion, and diced celery until evenly distributed throughout the macaroni.

Add most of the remaining dressing, reserving a few tablespoons, and stir again until the salad looks creamy but not soupy.

Cover the bowl tightly with plastic wrap or transfer the salad to an airtight container and refrigerate for at least 2 hours or up to overnight to allow the flavors to meld and the pasta to absorb the dressing.

Just before serving, stir the macaroni salad, add the reserved dressing (and an extra splash of milk if it seems too dry), adjust seasoning with more salt and pepper if needed, and garnish with sliced green onion if using.