The year is 2015. The Supreme Court has ruled in favor of same-sex marriage. Negotiations are about to begin on the Paris Climate Accords. Mainstream pop culture is essentially “Hamilton,” “Hotline Bling” and true crime docuseries. And Olivier Rousteing is at the H&M store on the Champs-Élysées, where he is starting to feel the pressure of the pandemonium outside. A policeman, he recalled, told him it was unsafe and he should leave.

Mr. Rousteing, the creative director of the French luxury brand Balmain, was making an in-store appearance for the debut of his collaboration with H&M. Just about every year since 2004, the mega-retailer has released at least one collection in partnership with a luxury fashion designer.

Much has changed in fashion in the past 20 years: the power players, the priorities of consumers, the trends and the platforms that amplify those trends. At least one thing hasn’t: the frenzy around H&M’s high-end designer collaborations.

Modern shopping has been shaped by this frenzy; collaborations have infested the landscape of fashion, from low to high. Just ask anyone swept up in the mess of Yeezy Gap in 2020 or Louis Vuitton and Supreme in 2017, or who has indulged in Dior Birkenstocks or an Hermès Apple Watch. Arguably, it all began with H&M.

Several of H&M’s limited-edition collections have sold out within hours. Thousands have lined up outside its stores, sometimes overnight, including resellers looking to flip the sold-out pieces on eBay.

When Balmain’s collaboration was released, one shopper in London described the scene as “dangerous” because of all the jostling. But for many fans of Mr. Rousteing, this was their first chance to buy his work — to dress in the curve-hugging metallics of the “Balmain Army,” whose generals have included Beyoncé, Rihanna and Kim Kardashian.

Balmain’s dresses typically cost more than $1,000, but at H&M, thanks to cheaper materials and production processes, a plunging sequined minidress that seemed to contour the body like celebrities of the era contoured their faces was $199. A velvet blazer with strong shoulders and intricate pearl-like embellishments was $549, as opposed to $3,000; a cotton T-shirt with Balmain’s logo was $34.99, not $395. The ad campaign was shot on a fake subway car, as if to emphasize that this collection was made for people who rode public transportation.

In a recent interview with The New York Times, Mr. Rousteing called the collaboration one of the “top three chapters” of his career. It was a validation to him, he said, proof of his appeal with the public, even to critics who may not have appreciated the young designer’s aesthetic.

“It changed my positioning in the fashion industry,” said Mr. Rousteing, then 30. “I was still ‘the kid’ in fashion at the time, and sometimes the press has been really tough on me. When they saw that the clients responded in such a humongous way, I think people started to realize that it was just not fireworks. My clothes, my vision, works.”

On Thursday, another designer will take a turn at this right of passage: Casey Cadwallader of Mugler, who has devoted much of his H&M collection to resurrecting archival designs, simplifying Mugler’s existing designs, or to making the sexy-alien aesthetic slightly more wearable. (Nipples may get three more centimeters of fabric coverage, for example.) In some cases, he replicated his “greatest hits” almost exactly, only using H&M fabrics and mass-production methods, as with a two-tone cropped denim jacket, priced by Mugler around $1,000 and by H&M at $299.

“Fashion is here to make people happy,” Mr. Cadwallader said. “The more people that the work can reach and make happy, the happier I am.”

Karl Did It First

When Karl Lagerfeld became the first designer to collaborate with H&M in 2004, people were in shock, said Shawn Grain Carter, an associate professor of fashion business management at the Fashion Institute of Technology. “Remember, that was the death knell for Halston, when he did a collaboration with J.C. Penney,” she said. Because of the collaboration, she noted, Bergdorf Goodman stopped selling Halston.

At the time (1983), the concept of mixing “high” and “low” was not yet in vogue. But at the start of the 21st century, it came to define fashion. The first lady shopped at J. Crew. Bloggers infiltrated the editorial elite, and emerging social media platforms gave rise to new style stars. Before all that, though, the designer Isaac Mizrahi made a splashy debut at Target in 2003. The next year, Mr. Lagerfeld brought his sleek collection to H&M.

According to H&M lore, Mr. Lagerfeld immediately said yes after being approached by Donald Schneider, who later became H&M’s artistic director. They knew each other already; Mr. Schneider had worked at French Vogue.

Mr. Lagerfeld’s participation signaled to other designers that they could follow suit. The next H&M collaborator was Stella McCartney, who was followed by a wide range of names. For every designer who went in the direction of streetwear (Moschino) and sport (Alexander Wang), others were inspired by deconstruction (Maison Martin Margiela) and the avant-garde (Comme des Garçons).

Ann-Sofie Johansson, the creative adviser at H&M, said the best-selling collaborations have generally been the “more glamorous” ones, versus conceptual collections, like Roberto Cavalli and Balmain.

“I think every designer is a little bit worried,” said Ms. Johansson, who has been with H&M for 35 years. “We all always tell them, there’s going to be a long queue, you don’t have to worry.” The designer Alber Elbaz was so concerned that he pulled up in a limousine outside the Fifth Avenue store to spy on the line for his Lanvin collaboration in 2010. He then took selfies and chatted with customers, Ms. Johansson said.

From the designer’s perspective, the upsides are clear: H&M offers intense exposure, including press coverage and a robust marketing budget. Prince and Nicki Minaj performed at a party celebrating Versace’s collaboration in 2011. Sofia Coppola directed a commercial for Marni’s collection in 2012, as did Baz Luhrmann for Erdem in 2017.

It can also be personally lucrative for them. The Times has reported that Stella McCartney and Mr. Lagerfeld were each paid $1 million for their collaborations. “And in many cases, beyond that, there’s some sort of a royalty or a revenue share on top,” said Marc Beckman, whose advertising firm DMA United has brokered fashion collaborations involving Gucci, LeSportsac and the N.B.A. H&M declined to comment on its financial agreements with designers.

Yet some designers, such as Rick Owens, have spoken out against working with fast-fashion companies, citing concerns over waste and disposability — an image H&M has spent years toiling to shed.

Here, the designers provide an upside to H&M: a “halo effect,” Mr. Beckman said.

“Some people will stay interested in the environmental concerns, regardless of these top-tier types of aspirational collaborations,” he said. “But a lot of people will look the other way so that they can get a piece of luxury.”

Democratization or Dilution?

A decade ago, Jessica Y. Flores waited in line overnight for H&M’s Versace collaboration at a store in Midtown Manhattan. She sat on the sidewalk, she said, recalling that it was so cold outside that people took turns warming up inside a nearby pharmacy.

She was there because she’d grown up admiring Versace. “But I am someone who was a first-generation American, and I come from a working-class family,” said Ms. Flores, now 36. “Buying high-end luxury to wear was not something that was available to me. I heard about this collection, and I was like: ‘Oh, I can buy this.’”

Afterward, she posted a video of her haul to her new YouTube channel. Today, she calls herself the “collaboration collection girl of YouTube,” having tracked 11 years of explosive growth in the mass-market collaboration landscape. She has made vlogs about buying bags from Fendace, sneakers from Nike and Tiffany and clothes from Target’s collections with Victoria Beckham and many more. (She has not received free or discounted clothes from these brands but has occasionally produced sponsored content for other companies.) (While Target’s collaboration program has continued since Mr. Mizrahi’s days, the company’s focus has turned increasingly to emerging designers rather than luxury brands.)

Ms. Flores has seen how collaborations have gone from mass-market efforts at democratization — a term used by most people interviewed for this story — to in-house drops led by high-end brands. When Gucci and Balenciaga collaborated with Adidas, for example, the pieces were sold on the luxury brands’ platforms and at their prices: $850 for a pair of logo-covered Gazelles or pre-worn-down Stan Smiths.

Lately she has noticed some negative comments about the accessibility of Mugler’s collaboration. “One of the phrases that keeps coming up, over and over again, is that ‘It’ll cheapen the brand,’” she said. Complicating matters is Mugler’s decision to offer some near-replicas of its core pieces.

“If the silhouette is so similar to the runway collection that you can’t distinguish why you would offer it at such a low price point, you are very close to the line,” Ms. Grain Carter said — the line being “the dilution of the brand.”

Quality and craftsmanship are what traditionally set low-end and high-end fashion apart. So what happens when H&M sells pieces that closely resemble the high-end versions, and made with more expensive materials than its regular lines?

“I was very much expecting the cheap substitute for every fabric that we normally use,” Mr. Cadwallader said of the development process with H&M. But in many cases, he said, the company was using materials similar to his. “It really looks like the real thing.”

Still, Ms. Flores is not convinced the “cheapening” conversation she has seen online is anything more than elitism. “Some people do not want the everyday person to have access to the things that they have access to,” she said.

As for Thursday, she doesn’t know whether she’ll try to shop the collaboration. Her favored H&M store in Queens closed last year. And the designs are a little too revealing and body-conscious for her current tastes.

But many signs point to another swift clearing out of every last pair of sheer leggings and all the corset hoodies. Because, as Mr. Beckman said, “H&M just dominates this.”