I was an editor offline and now online for a number of years.
You’re completely right about there being no manual.
However, some of the seemingly-modern obsession over user-centred design is merely an extension of what I remember concentrating on when editing.
We knew our reader(s) inside out. What they wore, where they worked, their leisure pursuits, their social and home lives… and this inevitably drove what we focused on publishing.
Instinctively, if you know your reader, you know if they’d like something and also what questions they’d want answering if they were presented with a piece.
I also always try/have tried to solicit as many opinions as possible and then make a decision.
In the 2016 documentary Absolutely Fashion: Inside British Vogue, there’s a great segment where Alexandra Shulman is presented with two covers of Kate Moss from which to choose.
She picks one shot completely against the advice of every other member of her team.
It goes on to sell monstrously well — naturally — and it’s all because she knows (as the editor) what will work better than anyone else.
In lots of ways, it’s instinct and a little bit of magic.