My knowledge is from consulting for fashion companies, so am not sure where to point you I'm afraid!
Most fashion retailers work with contract factories. They design a garment (or line of garments) and then put out a tender to a set of factories in their approved supplier list. The factories then bid to manufacture those garments. The decision-making criteria for the fashion companies vary but are usually some combination of quality on offer, price and 'time to store'/turnaround time. This tender process can happen dozens or hundreds of times a year for big, high turnover fast fashion retailers. Some of these retailers do have their own factories, but most don't.
The reason these retailers need to do this is because they're always trying to stay 'on trend' and have the latest, most fashionable items at the lowest cost. They're 'chasing the zeitgeist'.
Uniqlo, on the other hand, doesn't chase fashion in the same way. They have a stable set of core garments they always sell in predictable (and VERY large) quantities. This means they can negotiate longer term, much larger scale deals with manufacturers that work out MUCH cheaper for them. It also means they can go 'up the value chain' and manufacture the fabrics and patterns and tooling themselves, reducing cost even further.
Most fashion retailers work with contract factories. They design a garment (or line of garments) and then put out a tender to a set of factories in their approved supplier list. The factories then bid to manufacture those garments. The decision-making criteria for the fashion companies vary but are usually some combination of quality on offer, price and 'time to store'/turnaround time. This tender process can happen dozens or hundreds of times a year for big, high turnover fast fashion retailers. Some of these retailers do have their own factories, but most don't.
The reason these retailers need to do this is because they're always trying to stay 'on trend' and have the latest, most fashionable items at the lowest cost. They're 'chasing the zeitgeist'.
Uniqlo, on the other hand, doesn't chase fashion in the same way. They have a stable set of core garments they always sell in predictable (and VERY large) quantities. This means they can negotiate longer term, much larger scale deals with manufacturers that work out MUCH cheaper for them. It also means they can go 'up the value chain' and manufacture the fabrics and patterns and tooling themselves, reducing cost even further.