You can build them similar to factories.

引用自 Dawedws：

You can build them similar to factories.

It doesn't look like I can build those refineries that u use to increase steel production.. I can build the steel mill, but that doesn't increase that amount.. at least not on a 1:1 ratio..

Only building mills (steel, aluminium, ...) doesn't increase the production at all. You have to activate them in the energy/trade panel. On the left side ...

I still don't get it. I do not see anything in that panel that suggests "Using" the steel mills.

go into the trade tab. On the right panel in this menu you see two tabs. one for energy and the other for your refinieries. You have refineries for steel, aluminium, rubber and fuel (by coal). You see the max amount on the right of each, these are the buildings which your country owns. you can build all of these. To get the ressources out of it you have to add the refineries to each production tab. just like in the regular production menu for army equipment.