From Archon Arcana - The KeyForge Wiki

After a card with a “Scrap:” ability is discarded from a player's hand the active player resolves the card’s “Scrap:” ability. “Scrap:” abilities can resolve in the discard pile.

If multiple cards with “Scrap:” abilities are discarded simultaneously, they are resolved one at a time in the order of the active player's choosing.

If an ability discards a card from a zone other than a player’s hand, any “Scrap:” abilities on the card do not resolve.

KeyForge Master Rulebook v17.1 September 2023