I have no specific information on that, but you can be sure that many parts are made by specialty machine shops and they're made in large production runs. Some of the "manufacturers" contract the machine shops to make them.
I know for a fact that there is a machine shop somewhere in the general are of Daytona, FL, that actually fabricates M16/AR bolts and carriers from raw billets. I've seen barrels full of rejected parts in various stages of completion at a scrapyard in that area. NO, that scrapyard won't let that stuff get out due to contract stipulations, and I'm not willing to run reject parts in one of my guns anyway. They MIGHT be safe, but then again, they might NOT.
I speculate that the machine shop that mades them MIGHT be Knight's Armament in Titusville, which isn't all that far away...but that's only speculation without a shred of evidence other than location.
I AM sure that Knight's machines their own bolts and carriers.