Maybe I’m better at this game than I thought, or you guys are trying to pad my ego :-)
Pretend you have 1 army and 1 idle factory & you tell that army to add 20 Archers: these will queue up on the factory. If you want to make 1 Hermes (maybe to protect your home base) and click build on that same factory, it will get queued up behind those Archers in spot #21.
If you have a number of factories, the game decides which one does the building. I liked this mechanic because late in the game: you have a lot of factories and resources & don’t really care where the new units get built.