Until we are told we just have to assume, which sucks, but then nothing's perfect. The Is Clans may well be trading for material and maintenance from thier host states, but that still leaves the Falcons/Wolves and maybe Horses still unanswered.
As to the homeworlds, it is either large knowledge gaps, or a super specialised economy, where certain Clans trade certain items with the other Clans to ensure a balanced Touman.
Until I win the lottery and buy the rights to BT, it will likely remain a mystery. However, when I do win, you and MadCap will be first assigned to the Industries Review series.