An interesting video I came across recently came from The Gun Collective. They took a tour of the Daniel Defense factory, and spent some time showing the hammer forge machine in operation. I briefly discussed how hammer forging works on my guide to AR-15 barrels, but this is a nice visual representation. The key part starts at 1:27 and ends around 6:27 (though the entire video is worth watching).
The remainder of the video shows the aluminum manufacturing and assembly processes that DD goes through. DD is a known quality company, with prices to match. So, again, you have to ask yourself what steps other companies are skipping in order to give you that cheaper price.