While looking at the brand might be a good heuristic to rely on in the short term, the temptation is too high for vendors to take advantage of their brand power to offload cheaper fabrics for higher margins, I'm looking at you H&M and UNIQLO ...
They either had to dramatically increase the price or lower the quality of their stock. It is pretty obvious which choice they made. You get what you pay for.
They do have some polyester crap, but they are better than most at having 100% cotton options.
As you've said, you really can't judge by the brand.
Some of these exist now in the form of (maybe) physical store (or online-only) plus youtube personality, of course.
You can still get high quality or at the very least 100% Cotton clothes there but you'll have to seek them out and they know people will pay a premium for them so they tend to be 2x or more the price of the popular Airism t shirts for example.
I did give up entirely on trying to find outerwear there that was at least roughly >80% organic materials like cotton or wool which was probably my biggest disappointment. You can find nice basics with good quality fabrics at many brands. But Uniqlo 10 years ago was my favorite for wintertime because they're one of the few that had affordable coats and outerwear that made use of real wool + down with good quality lining, excellent heat-tech jackets that used a great blend of breathable fabric + artificial ones to keep you warm but not sweating. I've worn an Uniqlo duffel coat, peacoat, and several jackets every year for the better part of a decade and they still hold up excellent besides some pilling on the coats that I haven't fixed yet.
They don't even really seem to carry proper coats anymore in their stores nor decent jackets, everything seems like the cheap polyester fleeces and puffer coats that everyone else has.