At gun shows and many retail outlets (that sell "parts" for AR's) many say all AR parts are made by a few companies. And many "builders " use parts from these suppliers stamping their logo on them calling them MIL SPEC. In many cases this might be true as most look alike. But if you take the time to read and inspect mant different manufacters products you see differences . Finish , shell deflector, how the lower is made to not accept a full auto trigger group, forged vs cast , Steel the bbl is made from, fences vs no fences, forged vs powder controls , type springs etc.. If all was equal there would be no need for things like wedges , offset pins etc.. The bbl is the heart of the gun , military bbls are 4140 I think and those on the civilian in many cases are 4130 ( I may have the numbers backwards ) . A military spec bbl is stronger and harder to take abuse such as sustained auto fire and knocks getting out of choppers and vehicles.
So if you can don't rely on looks check the specs to see what you are really getting. Does the gun have a single stage trigger ot two stage one ? What is the finish other than black ? Are the internal parts domestic or imported ?
Pick up a shotgun news it has a worth of parts for sale with discriptions , use as a learning guide . Sights are a big thing if supplied are they composition or metal . These things are hard to see in a picture and sometimes in hand.
Best to decide what you want and but the best you can.