Continuity mistake: When Adriadne and Cobb sit down, a newspaper stand explodes, but in the next shot it's still standing.
Continuity mistake: As they are sitting around, planning the next job, Cobb is wearing a blue shirt, of which he rolls up the right sleeve repeatedly.
Continuity mistake: When Arthur is shot in the head, at first there's no bullet hole in his head, but in the next shot there is.
Chosen answer: The details are never explained in the depth, but all the dreamers are connected via the briefcase, so it seems likely that the briefcase acts in some way as a networking hub for them to share the dreamspace.
Tailkinker ★