Some companies physically build their own, such as Asus, Foxconn, Sapphire, Palit, Galaxy and a few others. Some do not own actual manufacturing plants thus contracting out, such as BFG, EVGA, PNY, Visiontek and maybe others.
It's like this: How many PSU companies build their own? Okay, Seasonic does, as does FSP, Enermax and maybe a few other lesser known brands of lesser availability or popularity (Etasis? HEC?). However, most "brands" on the market just leverage their marketing acumin with the manufacturing prowess of some Asian CEM for their products. This includes Corsair, Antec, Silverstone, Ultra, BFG, OCZ, Thermaltake, even PC Power & Cooling among others.
Uh, what? Dunno which "very cheap company" you are talking about, but the higher end reference cards all come from Foxconn or Flextronics, no matter which Nvidia partner stickers their name onto the finished product. Non reference can either go pseudo high end (with dubious results), go completely low end ("cost down") or just slap a fancy cooler on it and call it a day. This is true of pretty much all partners.