This article talks about how they produce their fabric, but the thing they realised super early is how valuable _vertical integration_ can be in fashion retail. They own the end to end process, from raw material purchase upwards.
They can do this BECAUSE of how limited their selection is, if you're a fast fashion retailer you always needs new materials and new patterns and new tooling so it's incredibly hard to chase vertical integration and strong unit economics. They get savings by pushing that risk and RnD onto dedicated manufacturers in places like Bangladesh. It's 'just in time' manufacturing in a pretty extreme sense. Because Uniqlo have so much less cardinality in their stock they can focus and optimise for it. It also enables them to control quality in a way few others can.