As I said, I have Wildfire 6 maps, mercs and story installed on top of Ja2 1.13, running under Linux perfectly. Its not current 1.13 build though, because I found nothing interesting what they add, like artillery etc. Ja2 needs a complete rewrite with crossplatform moddable engine, 1.13 is really as far as the original JA2 code could go, but its garbage on modern systems with high-DPI etc.

In contrast to original Wildfire 6 sold here,.. in my version I have fully configurable gameplay in every aspect about what happens in game. Most importantly, the gun spawn rates, availability of Bobby Rays and so on. And ofc much more guns.
Biggest Wildfire 6 issues are the strange difficulty, enemies shooting with pointing fingers, useless Bobby Rays and hard freezes from time to time. Thats all fixed. Theoretically anyone can make the same thing I have, just need to have JA2, get WF maps mod and mix the proper JA2 1.13 version into all that.

AFAIK Brigade is -the- JA3 (it was even sold as JA33/JAZZ in RU region), that was funded by the ja copyright holder, but then disagreed on what came out and cut the payment.
AFAIK Wildfire was originally a mod to JA2 (WF version 5), carefully written by two dedicated Russian players, who then tried to get SF co-fund them into the improved JA2 with bigger maps etc, but they got screwed, so they got that Zuxxez company running and released WF6 that runs on JA2 codebase with all those improvements, but unrelated to SF... Maybe they wanted to get some cash through digging up the code/game and disagreed with SF about how much each should rob off the end product, who knows. But it didn't work out very well in the end.

But Steam requires one to own Wildfire to buy JA2 classic. One game requiring another to buy but isn't a expanse on top of it? How does that make sense? I've never seen that before.

Not sure about it. I own Wildfire as an standalone on Steam but not owning JA2 gold

That could very well be. I read about it not long after WF6 was released. They got screwed by SF, but I have no idea what kind of deal they made with Zuxxez/TI. At least we ended up with a nice addition to the JA franchise.

Usually it's called an add-on. They were quite common in the 90s/00s. In most cases they were true to there name and really added something to a game. Unlike todays replacement called DLC.