I get that they ask other people and they agree, but how do they collaborate or do choruses? Do they record their vocals and send the vocals without the music to the other person or something else? Sorry if this sounds stupid '^-^
Because if you send in vocals with music how are they supposed to mix it to sound decently? They would have to first remove the background music from your track and then mix it together with the other vocals.
And I think you'd send them in without any editing. So that the mixer can put them together himself.