In Civ 6, pretty much everything is handled through the new Modifiers system. You don't make a trait and add it to a civ or leader; instead you make a new modifier and attach it to the civ or leader (or unit, building, policy, belief, etc). The modifier's type determines what group of things it acts upon, the effects of the modifier on said things, and what arguments are needed. The only way I know so far to find these things is by looking at the base game's files and reverse-engineering the modifiers.
For your effect, you might need a new modifier type (not entirely sure that's a thing you can do as of yet, really), attach it to your leader/civ, have it affect all the player's cities, and use whatever effect the Ruhr Valley has to apply the percentage bonus to production.
Sounds complicated, because it is. There are a couple resources for learning the new modifiers system already on the boards, but I'm on mobile now so I can't easily link them. Once you get a handle on it, what I said will probably make a lot more sense.
