You don't. In fact I'd argue that having access to the details makes you more likely to both choose the wrong garment and rationalize that wrong choice.
> For someone that sweats a lot in hot & humid climates, why does one white cotton shirt show the sweat, and another doesn't?
Branch one of this answer includes all kinds of details about thread count, thread length, thread quality, and all kinds of other details for you to get lost in.
Branch two: don't wear cotton in hot & humid climates.
> Why do I feel cooler with a thicker cotton shirt than a thinner one?
Branch one includes all kinds of wonderful details about the way in which cotton wicks moisture but does a poor job of evaporating that same moisture. (Your thin cotton shirt sucks up your perspiration to arrive at the "moisture traffic jam" much sooner than your thicker one.)
Branch two: don't wear cotton in hot & humid climates. Synthetic, bamboo, and even light merino tee shirts will all perform so much better that the difference between thin and thick cotton will become thermal noise by comparison (pun!).
Seriously, don't learn the details. I can't tell you how many times I've had pointless conversations about the differences between various proprietary waterproof breathable membranes to people who are using the rain jacket exclusively in the Southeastern U.S. (Hint: none of them will do a decent job of being breathable there. So just pick one, unzip the vents under the arms and get on with your day.)
I seriously had to LOL at this. When I first moved to Japan, I was clueless. I live in a particularly hot and sunny part of Japan. During the rainy season, we sometimes have rain every day for a month (like this year :-P). It took having my clothes literally rot on the line before I understood this point. My predecessor (government program rotates foreigners to teach English) was famous for going to events with a shirt stained green from mould.
It's the very first thing I tell people when they come here: don't wear cotton. There are much better natural fibres for this weather. You can also have cotton synthetic blends and they won't rot. The clothes that work well in the UK and most parts of North America can't stand up to 2 months of 30 C plus at 90 percent relative humidity.
Having said that, there are a lot of traditional cotton clothes in Japan and I have found that they are surprisingly comfortable. However, they are expensive. Probably those details you are talking about ;-) Practically all of the cheap clothes I know of are synthetic blends.
I'd give one other small piece of advice: buy clothes in areas that are famous for the weather you want to feel good in. Anybody wants a nice, not-bulky, warm winter coat: buy it in Canada. You want a nice summer suit for 100 degree F weather? Taiwan. You want a sweater for those cold rainy days? Scotland.
That's a nice start-up idea.
I can't afford to travel the world to do my seasonal shopping.
For a summer commute from SF to Sacramento I'll bring a waxed cotton raincoat. After all, that's what all the people wear in that city that is famous for rain.
Or I can take a deep breath, admit to myself that I'm not a domain expert, and forge a relationship with someone who is.
This expert will use clues from the relationship to help me choose a garment. Maybe they find out I'm generally never in the elements for greater than 5 minutes. So they show me this dinky little windbreaker that is so light it can actually fit in my pocket. They explain how it isn't actually waterproof but it has a treatment that makes water bead off, and the ease of shaking it off and small size might be a decent tradeoff given my situation.
So now I'm carrying vastly less weight with a useful garment. I never would have chosen it on my own because my non-expert brain fixated on the word "waterproof" as if it were a boolean. Meanwhile, the domain expert measured the time it takes for water to penetrate the seams and found it plenty long enough to meet my needs.
> Anybody wants a nice, not-bulky, warm winter coat: buy it in Canada.
The current trailblazer is almost certainly Patagonia's micro puff jacket which uses a synthetic fill that AFAICT reaches the warmth-to-weight ratio of down. (Added potential benefit that the synthetic fill would continue to insulate if wet while down won't.)
Patagonia is a U.S. company, so you missed out on the latest greatest by choosing based on country conditions.
I'm being super finicky, and of course there are Canadian-made non-bulky winter jackets. But this thread started with someone wearing cotton in the summer heat. Such a person can just as easily buy a cotton hoody in Canada.
Anyways I don't need the nitty gritty details, but there should be some sort of consumer friendly information that would make it easier to judge things. Not necessarily what the experts use.