They use a fine metal powder squeeze it under somewhat high pressure in a die to form the shape. (Sintering is NOT forging) The powder has a binder in it so when its sintered (heated up to high temp) it binds together. Powdered metal parts are OK but I wouldn't make gears out of it which are under a lot of stress. Would be better off hobbing the gears and then heat treating them but that costs quite abit more than a powdered part. A gear machined from a forging or a solid piece of stock would be a lot better. Tungsten carbide inserts common in the machining industry is a good example of a powdered metal part.
Brazing is more of a term that describes a heat range. For example once you get over a certain temperature you are no longer "Soldering" a part together you are now "brazing" it together. You can braze brass, copper, steel cast iron and etc with the same low fuming bronze brazing rod and a oxyfuel torch.