Soulcasting

I think this question has been asked before but it was a while ago and I couldn't find any fleshed out answers. Now that I'm on a reread, and WAT is out, I'm wondering if someone who is better versed with the cosmere knowledge might be able to take a stab at this please?

*SPOILERS FOR AT LEAST ROW AHEAD*

So as far as I understand, soulcasting works by bargaining with an objects Spiritual Identity through the Cognitive Realm . I.e. you convince something to change its perception of itself by finding its bead in shadesmar. My question is, how does this work with the objects being soulcast that are native to shadesmar? Specific example being the obsidian blocks that are soulcast into grain in RoW? When you soulcast something usually, the object changes in the physical realm, not the cognitive realm. When they use a bead to make something in shadesmar its called manifestation, not soulcasting. So why can they soulcast directly in shadesmar? And where is the obsidian's cognitive self? Why does this not present itself as a bead?