The only store bought I ever purchase is Blue Ridge but their dressing is mainly Hellmanns mayo and preservatives. I only buy it once or twice a year so don’t worry about any health consequences; at the price they sell it at Fourth of July it’s like a gift from God! One of my favorite factory salads. But of course they’re not giving away any little secrets on their ingredient label, and I never worked in their factory so hard to say. (I’m assuming you’re not talking about private deli made salads?)
My brine, concocted from the deli I cooked for, is 1 cup mayo (to which I now add yogurt), 1/2 cup lemon juice and 1/2 cup Heinz white vinegar. I also do a splash of pickle juice (an old deli trick!) and a couple of Tbsp of sugar or honey for that sweet and sour flavor. Another big “secret” is soaking the undercooked macaroni in some milk before dressing it.
But if you don’t live in the NY area, this German style concoction may not be what you’re looking for either. You probably should say what region you’re from.