I get that this is just promoting a book but like what even is the conspiracy here? Where are they saying all this stuff went?

I believe the world trade centers had basements that went down 60+ feet so I always assumed the rubble collapsed mainly down into that. Considering buildings like that are mostly air/open space, that seems to make sense to me. Things like toilets and file cabinets were squished down to basically nothing from the forces of the pancaking floors.