Does OP mean what was happening mechanically with printing?
The short answer is rotogravure, photographic platemaking, and rich black.
The long answer is that magazines in the 70s and 80s (with the exception of National Geographic, which dragged its feet a little and still used a letterpress until 1978) were printed by a process called rotogravure, an intaglio printing technique in which the image (in this case, each page of the magazine) is engraved directly onto a copper cylinder and then rolled over paper to make an impression.
(Apologies by the way if any of this sounds condescending. I work in prepress, and my dad is a master printer, and I never know what people do or do not know about the printing process.)
Anyway, you'd have an engraved cylinder for each color—cyan, magenta, yellow, and black—and you'd roll the paper across each one. And then you'd have a finished page of the magazine.
The alternative to rotogravure is offset printing, which is very similar but uses an oil-and-water repulsion technique on an aluminum plate rather than an engraved cylinder to transfer ink to paper. Offset printing is great for short runs when you want to print a small amount of something that changes frequently. Newspapers are usually printed offset.
Rotogravure, on the other hand, is best for very large jobs that require high quality and more consistency. Color magazines, catalogs, and wallpapers are all things that would be printed by rotogravure.
Rotogravure is also a negative relief printing technique, which allows for more sensitivity when it comes to grade, saturation, detail, dimension, etc. The cylinder itself holds much more ink. Offset, by comparison, is a positive relief printing technique, which means you can't get as much ink on the page. Plus, offset uses flimsy plates (designed to be very disposable). That's why newspapers can look more faded and be less consistent than a magazine.
Additionally, rotogravure uses alcohol as an additive. Offset printing uses water. Alcohol dries faster and creates much sharper halftone dots. Offset halftone dots, by comparison, tend to bleed and soften a little.
All of that is a bit of a preface to the important part:
In the 70s and 80s, most of the printing process was still done photographically. Negatives were developed in a dark room, separations were created by actual physical filters, halftones were created by physical screens, and every page of the magazine was laid out (called a "paste-up," literally with paper, acetate, and paste) and transferred onto printing plates. In offset printing, the printing plates are made by a large camera called a platemaker, which is kind of like a fancy photocopier that prints on special aluminum sheets. In rotogravure, the printing plates/cylinders are etched by chemical or laser using an engraving machine.
Because everything was done photographically, people were highly involved in every step of the process, which meant that printers—the actual people—were very skilled and highly individual. They had their own ink mixes and densitometric specifications, their own machine settings, and their own printing press calibrations. A printing house would therefore have a signature look. And if a lot of big magazines are printed by one publisher (for example Condé Nast, which at one time printed many magazines like Vogue, The New Yorker, GQ, Vanity Fair, and others, but which has since converted some of them to digital only) then all of those magazines would use the same presses, the same papers, the same ink mixes, and the same photographic process.
Thus, they would all look the same.
So let's talk about rich black.
Magazine paper is unique because it's very thin gloss paper. And that's what you want with rotogravure printing, which is all about consistency and sharpness. Newsprint is quite soft and bleeds ink and isn't the most durable stuff. But magazine paper is basically the cheapest nice paper. It's as cheap as paper can be that can still hold a lot of ink. Such as rich black.
In four color process printing (also known as CMYK) there's cyan, magenta, yellow, and black. But 100% black ink by itself is not very dark. It's quite flat and grayish, which is perfectly fine for text (think newspaper headlines), but not very nice when you're trying to print high quality fashion photos in a magazine.
In order to make black look as deep and rich as a photograph, a small percentage of each of the other three colors have to be added to it. Like so:
These days, a rich black mix is all done by computer, and it's all quite standardized. But back in the 70s and 80s, a master printer in a printing house would have their own preferred mix of rich black. And they would use that for all their color printing.
And all of that is why those ads look like that.
I probably explained too much.
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