Question: how do you unlock the forges in destiny 2?
Answer: You can find one in Exodus Black near the rock formation West of Failsafe. Wait for him to spawn, kill him, claim his drop, and then head North of the Well of Flame on Nessus for another mission. Finally, track down the Forge by following the in-game prompts, talk to Ada-1, claim your weapon frame, and get grinding.