History of Resonant Alloy

Resonant Alloy is an uncommon crafting material introduced in Season 16 with Destiny 2: The Witch Queen, capped at 80. It is spent on weapon crafting to unlock intrinsic and enhanced intrinsic perks; raid weapons use Harmonic Alloy instead.

Acquisition

Dismantling Legendary weapons (occasionally Rares), five from Throne World Engrams after Rank 30 with Fynch, and some quest rewards. Shacknews also says to dismantle Legendaries.