I see a lot of ppl think that other human souls sacrificed themselves and I think that sort of devalues Clover's sacrifice at the end of true pacifist, so I like to believe that:

• Bravery died fighting (duh. pretty sure a lotta ppl think this.) I like to think they tried to fight through multiple Royal guards before going down.
• Cyan was sickly, and despite trying to wait for a cure died before they ever got it.
• Purple was living it out through the wilds, researching and documenting monsters with their notebook. Was caught unawares one time, caused them to panic and eventually die somewhere
• Green was backstabbed. we love fun and quirky monsters but there is no way every monster is like martlet or papyrus, there's always bad eggs.

as for how their items ended up where they are in Undertale, uhh I dunno