Having worked for some years at a book publishing company, a few thoughts. Even when writing fiction, a lot of research is required. The process begins with an idea which is then supported, and augmented, by real-world information. The company I work for has a library with various books on various subjects. Generally, I'm asked to locate the best books and then they come in. Although I work primarily as an editor, I need to be familiar with what I'm reading. It's not just spelling and grammar but a cooperative arrangement where those things learned, combined with what is known by those involved, produces the best result. Tom Clancy is read by specialists and non-specialists.
Of course the non-fun parts are little known. The public only buys the finished product and just assumes that those behind the scenes have acquired their arcane knowledge somewhere, along with their skills. Crowd-funding won't work with something that is poorly done regardless of what it is. I regularly scan one major crowd-funding site and the high number of unfunded and little-funded projects is the norm. Those started by people with obvious skill usually get funded.
And prior to the internet, how did things get done? Whether it's fiction or nonfiction, the big 6, down to 5 now, have to be able to tell good work from bad, and know what the market wants regardless of genre. They've had years to compile historical information regarding what sells and why, and keep adding to it. This is popular now, tomorrow, perhaps something else. It's not quite true to say that the various markets/genres are cyclical but broadly speaking, they are. The public buys this, others produce more, the public tires of it and something else is published. Vanity presses existed in the past as now. But imagine the pre-internet publishing world as being the size of the Great Lakes in the US. Then imagine that number swelling to the size of all of the oceans combined post-internet. And believe me, the new tonnage is mostly bad since the delusion of freedom from editorial "interference" has created zero to very small sales for the newly "liberated." Those who do have some skill are going along and doing well, but newcomers who seem to operate in a 'there was no history of publishing prior to the internet' don't know how to do another valuable thing - promote their work. Fiction or nonfiction. Books are languishing after a few social media posts, and "Why isn't my book selling?" posts.
Large, established military book publishers are still out there. They have a built-in following. The amateurs may learn but many do not. Posting on various forums and asking very fundamental questions usually do not get answers from professionals. But somehow, again, that big "somehow," more books keep coming out from here and there.