I come from a Caribbean island but I married a corn fed gal from northwest Iowa. If I may take a moment of your time, I would offer a defense of Velveeta.
Cheese results from acidifying milk which causes the proteins to clump together trapping fat in a net like weave. When heated, the fat renders out and you get oily goo dripping out. (Why some pizza and burgers made with "real" cheese can be greasy.) The fat in cheese also makes it less shelf stable for storage or shipping.
By adding Sodium Citrate to "real" cheese you can displace some of the Calcium which exists in milk and it makes the protein net more flexible. (Now it may be Sodium Phosphate which is just another type of salt and other milk proteins like whey but not, "plastic".) Heating this looser net allows it to melt more easily.
"Sodium citrate comes in and replaces the calcium ions with sodium ions, which are less positively charged (sodium: +1; calcium: +2). They thus make the caseins more negative. That means they’re less hydrophobic and more soluble, with the result that caseins now bind each other much more weakly than they did in cheese. They rearrange themselves into a looser, springier net, forming a substance that’s halfway between cheese and milk: the proteins are insoluble enough that their net can still trap fat molecules, yet soluble enough that the net is flexible and can withstand the heat rising up from a hamburger patty. Food scientists at Kraft describe it as partially reversing the process of making cheese." http://discovermagazine.com/2011/sep/16-the-secret-of-velveeta-how-cheese-food-is-made
Americans eat more, "cheese food" that real cheese and it stores longer and ships to market more easily. Copy and paste the link above and don't hate Velveeta. My Iowa girl likes her queso dip.
Well, there’s processed cheese like Velveeta which is real cheese modified with sodium citrate as you point out. By law it can still be called cheese on the packaging. But there’s a lower level of cheese that’s not really cheese (or has a lower percentage of real cheese). It can’t be called cheese but instead is called “cheese food” or “cheese product”. The question is, is McDonalds using processed cheese or something more questionable in its cheesiness.
BTW, sodium citrate is readily available and can be used at home to make Velveeta style cheese from regular cheddar or whatever. It’s sort of the best of both worlds...you get the uniform meltiness of processed cheese but with the flavor and simple ingredient list of normal cheese. Perfect for making queso or melty nacho cheese:
https://www.huffingtonpost.com/2014/11/06/nacho-cheese-sodium-citrate_n_6108794.html
Apparently you can also use gelatin (and lots of interesting processed cheese info at this link):
Velvets has its uses and some may indeed include mixing it with food. But I doubt mcD uses Velveeta. Just a guess whatever they use comes from a plastics factory