Yeah, I'm not sure how that's even supposed to work. In a traditional RTS, massive infantry rushes is a basic tactic. But in DoWII you don't get more units and it's always just a handful of soldiers. In the campaign, you mowed down huge waves of Orks and Tyranids, which is how it's supposed to be according to the lore, as I understand it. Do you get tank factories as IG, hatcheries as Tyranids, etc...?
You mean in the Retribution campaign? You can start a mission with a group of 4 heroes or 1 hero and 3 honor guard of regular units. Then you can capture 2-3 types of HQ buildings that give you access to various number of units and you can spend the resources you find during a mission to either revive/upgrade the heroes for the duration of the mission or buy more regular units. There's also abilities that let you summon units sometimes. Tyranids and Orks have passive abilities tha reinforce and/or summon units, as I recall.