Any discussion of metallurgy in terms of Yamnaya or more generally the steppes is very time dependent.
In the earliest period they imported copper objects, usually from the west.
As time went on they made copper artifacts of their own, using copper from the west, but they were simplistic and inferior.
As yet more time passed the steppe peoples did adopt bronze metallurgy. That seems to be after the time of the earliest Corded Ware movements, because they had only copper in the beginning.
Let's not try to re-invent the wheel, people. This is all well-known.
We discussed it here at great length back in 2014.
Steppe people became good metallurgists, i.e. Srubnaya and Sintashta, for example.
https://www.eupedia.com/forum/threads/30625-David-Anthony-on-Metallurgy