The forges on the open world can be used by anyone. Let's take woodworking for example: when you want to create a weapon, you have to select the weapon icon in the upper right hand corner. When you want to create a shield, you have to select the armor icon next to it.

The forges in the open world are a little different. In the woodworking table, you will notice there are now 4 icons, two of which have a triangle in the background. The symbols with the triangles represent the items you can create with the special attribute associated with that forge. In order to unlock the ability to create weapons or armor with that special trait, you have to research a certain amount of traits from that weapon or armor type.

For example, if I want to create a bow, a helmet, and a pair of boots that will give me + 100 armor when all 3 items are equipped, I first have to research 3 traits for the bow, the helmet, and the boots. Then I can go to the open world forge that grants the bonus to make the items.

Keep in mind, not all forges have the same requirements. In the low level areas, you need to unlock 2 traits, medium area 3, harder areas 5-8.

Like I said earlier, though, if you don't care about the special bonus and you just want to craft a new piece of armor that will get you by until you can head back to town, make sure you select the icon without the triangle and you will be able to craft normal weapons and armor in the open world.
Hope this helps!