given that the bespoke companies (Inditex, H&M) are regularly selling off half their stock at 50% off and meanwhile looking into insolvency every few years, I think you overestimate their execution. Producing a shirt probably costs 40cts (if at all) at enormous costs to communities in the disenfranchised parts of the world.
I doubt that the "brands" do any "finegrained" optimization, it's "make shirt or have no food"; the actual subcontractors probably do, but not in any rigorous way except introducing slavery.
They are viscously price-sensitive industries, always flirting with commercial problems, running on very thin margins with ugly working capital requirements.
These are exactly the kinds of companies that will use nearly-free AI instead of real models.
And FYI, they are excessively optimised for efficiency, more so than most industries. When an item doesn't sell, it goes on clearance right away. They adapt faster than any other industry to trends, down to the micro level. 'Home Base' knows exactly what is going on in every store and everything they make, cost & labour is a key consideration: 'this kind of seam -> this kind of cost' , 'this pattern doesn't fit well onto sheets of material leading to XYZ amount of waste, implies ABC extra cost'. Their online data is mapped to local inventories etc..
That's the only way 'fast fashion' exists, and it's a function of their scale, reach, adaptability etc.. In many ways, they are 'exemplary'.
I doubt that the "brands" do any "finegrained" optimization, it's "make shirt or have no food"; the actual subcontractors probably do, but not in any rigorous way except introducing slavery.