I apologize for asking what appears to be a very basic question, but several recent posts have confused me. The attachments don't have to be played on a character of a specific sphere unless the cards says so correct?

That is correct. The sphere of the card dictates the resource sphere you use to play it. The text of the card will speciphy if there are any restrictions on who the card can be played on i.e. Hero only, Hobbit only, Lore Sphere only etc. If the card does not speciphy a restriction there is none.