When it came to writing the background material, the troop descriptions, timeline stuff, and colour text I had a good head start. The Draconis Alba galaxy has been in development since the late 1990's, and I was involved back then in its original incarnation as Void.
John Robertson and I were working together on another project back in 2008, a 10mm scale fantasy system that has yet to see the light of day, but while we were working on that, we wondered what would happen if we applied a similar set of rules to the Urban War/Metropolis universe, and thus Age of Tyrants was born.
We had to decide how this new game would fit with the previous canon and had a chat about the sort of image we wanted to project with Age of Tyrants. It was to be very much about the heavy machinery and mechanised warfare, which lead to some interesting design decisions for the background.
We decided that to keep things tight we would stick to four factions, which meant Koralon (the only alien race in the Draconis Alba setting), as well as the new factions introduced in Urban War and Metropolis (Triads, Gladiators, etc) wouldn't feature. That lead to the decision to set Age of Tyrants in the past, before first contact with the Koralon, as a sort of prequel to all our other stuff. Another decision that fell out of this approach was that we decided Syntha would field wholly robotic forces--all SPOMM, no prosthenes.
The galaxy is roughly based on the Milky Way, and we're looking at a massively advanced human race that have colonised most corners of it, so classic sci-fi influences for this sort of set up would be Asimov's Foundation, Iain M. Bank's Culture, the Federation from Star Trek, Frank Herbert's Dune, etc. That's the level of tech and human expansion we're talking about.
Each of the factions have an identity that we always try to stick to when it comes to naming conventions and general background feel. Ranks and unit naming conventions are unique to each faction, retaining the feel of them, but as be have a one size fits all command structure with the platoons, companies and battalions, certain liberties had to be taken with these to fit the rules.
The Viridians are are most inspired by a modern NATO army, more specifically a US one, so when it came to naming the vehicles for the Age of Tyrants we went for a mix of animals and mythical creatures one might associate with North America, so we've got stuff like Coyote and Bigfoot. Junkers are obviously based on ancient Rome and the vehicles, weapons and platoon names tend to be Latinised, e.g Octoris Incendia and Draco. The Syntha have an ancient Greek feel to their naming conventions, mixed with a bit of techy, alphanumeric jargon. Finally VASA are Soviet inspired, with a few nods to Russian folklore such as the Alkonost and Triglav in there..
Now, before anybody says cultural appropriation (possibly a stretch where the ancients are concerned), I don't think there's anything wrong with this approach. It's only a problem if you do something that encourages discrimination or negative stereotyping of a particular group, which we categorically don't do. Certainly with systems like AoT, I try not to paint any one faction as either the good guy or the bad guy, they all just have differences that grind against one another and cause conflict (sci-fi tends to be more ambiguous like that, though fantasy can sometimes be a bit more absolute). Anyway, I've been using it years to come up with game and fictional backgrounds and it's a very useful tool for world building, because there are elements, shorthands if you like, that readers instantly recognise and it helps them understand and buy into the world.
Moving swiftly on, when it comes to writing colour texts, there are various levels of this. There's out and out fiction, which is purely to entertain and flesh out the detail of the gaming world, and that's personified by the timeline articles which give a flavour of the galactic instability around the Age of Tyrants and allow me to exercise my political sci-fi interests, about different types of governments that may be attempted in the future, and why they come into conflict with one another. Then there's a sort of intermediate type article, which illustrates gameplay in an entertaining snippet of short fiction, and finally there's descriptive text for platoons, which are intended to give the reader an idea of how their bases will function in a game, but presented in a colourful way.
Anyway, I hope that gives you a little insight into some of the processes we use when writing all this mad nonsense.