I personally always compared them to the Annas from the Fire Emblem series who all know each other across like dimensions and no one ever questions it.
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They make a big deal of you needing to make a pact with Feo Ul so they can bring you stuff from your retainers in the Source.... yet you can send your retainers out on ventures to gather stuff found only in the First.
My retainers have brought me gear from ShB dungeons, including Mt Gulg. I choose not to ask how.
Headcanon; Feo Ul gave it to them to give to me just to make me wonder.
I could totally see Feo Ul doing something like that. Along the same lines, the first few sets of gear you can craft use a mix of mats from the Source and the First. Makes me wonder if you were just inventing stuff on the fly by throwing together whatever you had in your bag at the time.
You fused with Ardberts soul and now you shall have the material knowledge of the first. Then by using your knowledge of the source and by brainstorming (which means getting in the head again and again during dungeon runs) you shall now possessed the power to craft mixed products of both worlds.