Regarding legendary crafting
Any of the legendary precursor crafting processes involve making ever refined versions of the weapon and needing to salvage the previous stage for a mat to include with other items to forge the next stage.
I have not tried making Eureka so I don’t know the particulars on it, but you should get a recipe for making the next stage that lists what you need, and if one of those things is an item whose tooltip is “salvaged from x” then you need to salvage x.
as donari said. ive made the spear and working on the shortbow and it works like that. you craft a weapon, then salvage it for its ‘essence’ which is a collection item for the next stage. you do this 2-3 times.
‘Death smiles at us all. All a man can do is smile back.’