Ok, I'm not sure you've got a grasp of the hive (hell, I barely have a grasp of the hive and I've been trying to wrap my mind around it for over half my life now). It is more of a layer cake than most think of it. You have House Helmawr on top, then the Spire, Hive City where the major house are, the Underhive where all the action takes place and finally the Sump (see picture below). Pretty much all the gangs live and fight in the Underhive, house or outlaw. Now where they come from is a different story and depends on the outlander.
Spyrer: These are high born. They come from one of the spire houses. They are just down in the Underhive for a short period to prove they are cool or similar.
Redemptionist: These are just religious nutjobs. Think KKK, ISIS, or Westboro Baptist. They come from wherever and just gather under a David Koresh or similar deluded, but charismatic fanatic. Probably mostly made up of Cawdor who thought the church in the House didn't go far enough.
Ratskin: Come from the abandoned badzones in the Underhive and Sump. Since they worship the hive itself, they will often be found protecting Archeotech Hoards.
Scavvies: Come up from the sump to wreck havoc in the Underhive.
Ash Waste Nomads: These are a newer addition and the only one that specifically comes from outside the hive. In their first incarnation they were required to have some sort of hive wall breaching weapon like melta bombs or Krak missiles.
I'm sure there are others, but these are the major Outlanders. Hopefully that clears it up some.
