A lot of the parts found in AR's do come from a few places only.
Like uppers. Many companies use the same forge for uppers as a source. Colt, Bushmaster, and even the junk M1S kit guys have used the same.
Same goes for bolts, carriers, trigger parts.
That's not what matters. What matters is what that company does with those parts after they get them.
For example, Colt proof load fires and MPI inspects each and every single bolt. Bushmaster is said to do batch tests. Others do nothing at all.
CMT makes Colt's lower parts kit. But I can tell you right now, that the trigger parts in my Colt are absolutely not the same quality as the CMT parts I've used in other ARs. Colt hand selects the best finished parts. They test them. They finish parts better and smoother. A Colt employee told me the same thing, and I believe it as I've seen the difference with my own eyes beforehand.
So like I said, they might come from the same factory, but they are not all assembled the same, finished the same, tested the same, inspected the same ....
Also, most forges build to your spec. If you order one grade of steel, they will produce in that grade for you. If you order something cheaper....