The presentation is a bit confusing, but if you sit down to think about it for a moment, I think it makes sense.
Basically, when the Octopus Devil was unleashed in its big form, it traps the Aging Devil with its tentacles (that's why in the darkness Aging Devil finds itself in, we see that the walls look like tentacles).
Inside the darkness, black Chainsawman still awaits and from his mouth comes the MREEE guy (whom presumably the gang put inside Denji so that he shows up in the outside world).
The MREEE guy grabs the Aging Devil and is pulled back into black CSM's stomach again. But at the same time, they come out of Denji's mouth in the Aging world.