If feferi created the dream bubbles after she died where did all the dead beta trolls hang out?

Feferi asking the Horror Terrors to make the dream bubbles occurs very early in the timeline of the Furthest Ring. Time works differently there so events that seem to occur at a certain time to us may have come about differently in the Furthest Ring. For example, Rose and Dave creating the Green Sun occurred extremely early in the timeline of the Furthest Ring because once you enter the Furthest Ring time begins to shift in odd ways. So from Feferi’s perspective the dream bubbles are new but to the ghosts living in there, the bubbles have always been around.