One part of world-building I tend to leave until last (unless it is an important element in the story) is fashion. What do people wear? What are the garments called? Do I use the present-day terms? Do I make something up? And what, according to the society, is considered fashionable?
The first part of my last book, Talking to Trees, was set in our world with a 13-year-old who considered herself fashionable. I work at a university, so I'm used to what college students and recent graduates from high school think is in fashion (more on that later). But for trends among the younger set, I had to consult those more expert. That included my niece, some friends' daughters, some teachers of that age group (including my younger sister and another niece) and parents. For visual aids I could consider shows aimed at that age group, always keeping in mind when the show was filmed. Styles change quickly.
Some styles don't change quickly enough. Bare midriffs have mostly faded away from the college campus (though that could be due to the cold winter temperatures here). The pajama bottoms style is still strong, as is the 'almost falling off' jeans for males and extreme low riding ones for females. There's always the discussion in the news of the "wardrobe malfunction" for low tops but I never hear any talk (except among faculty and staff) about how uncomfortable it is to walk behind those underwear exposing practitioners (or, worse still, to see the back view seated in chairs). I guess there's a story or two there.
Another trend is whether to wear the shirt/blouse tucked in or untucked. But that decision isn't often mentioned.
One could say that for present day clothing, the writer doesn't have to go into great detail. Jeans, tee shirts, sweatshirts, suits, coats, jackets, ties, dresses, slacks, shirts, blouses, vests, sweaters, skirts, boots, high heels - all are common enough terms. It's only when you want to point out how fashionable your character is that more detail needs to be used, but then you also have to consider how quickly that will date your story. Are the jeans ripped because the character has been in a fight? Poor? Or is it a fashion statement? And then there are regional differences. Do you wear tennis shoes or trainers or tennies? Running shoes, athletic shoes, gym shoes or a brand name? Do you wear a vest or a waistcoat? What about fabric? Fleece, cashmere, or silk would all mean different things fashionwise than wool or cotton.
For fantasy there are generic terms - cloaks, tunics, smocks, boots, sandals, vests, breeches, skirts, kilts, robes, gowns - that don't require much explanation or description of the details. Unless you want to get into the embroidery. Or tassels. Or magical reflecting properties. But do you describe a kirtle? A cheton? A cotehardie? A jerkin? Or do you expect readers of fantasy or historical stories to know what you are talking about? If you do use the wrong term, however, you can be sure that there will be a reader who will notice.
For science fiction stories, there will be protective suits, pressure suits and/or space suits to mention, as well as other clothing choices. Does everyone on the space station wear overalls (or coveralls)? What type of necktie is popular in your future society - Ascot, cravat or bolo - and are bow ties still cool? What will teens wear? Will there be different casual attire for those living in rural areas on the colony planet versus those living in the city? Do merchants wear different fabrics than pilots? Are there fabrics from other planets? Would your main POV character notice?
And, what does your nonhuman characters wear? Does your dragon wear a scarf? Jewelled talon covers? Do your furred aliens wear only a harness or trousers and a vest? Does your crablike alien police wear badges? Do you mention how those are attached to the shell? (I did in one story). On ships do your alien crew members wear the same uniform (with modifications) as the humans?
For YA stories (both fantasy and science fiction), usually the most important questions are 1) is there a school uniform, 2) is it the same for both male and female (and nonhumans), and 3) what modifications are allowed for the fashion conscious?
Because Jody in Talking to Trees is fashion conscious, I had to mention what the characters wore. Jody looks down on Jeanne, who wears jeans and a plain sweatshirt, and she complains when her brother Peter choses to wear a sweatshirt and jeans with multiple pockets and loops (which he fills with food and a flattened roll of duct tape). Jeanne and Peter know they will be walking through forests and across plains and dress appropriately. Jody, on the other hand - well, Jody's denim jacket is illustrated on the cover. There is a reason why leaves are growing on it.
Do you notice fashion choices in books?