Heavenfall, I see it a little differently, though I agree with the way it's progressing.
From what we've seen the last two episodes, the tall aliens, apparently the ones who are in charge, don't get their hands dirty. The get their skitters and mechs to do it all. The skitters, we learned, have been harnessed themselves. Wait for the upcoming "big twist" in which we learn that the skitters weren't always skitters. The harnesses remade them, and we're seeing, through Ben, that the human children who have been harnessed are physically changing too. They may end up as skitters themselves, or something else, but in either case they will become loyal drones themselves.
If we work this back, I'm willing to bet that these tall aliens move from planet to planet, wipe out organized resistance, and harness the young to expand their stock of thralls. Using local materials, they construct, perhaps, additional ships to house the new thralls, and then move on to the next planet, or perhaps split up and head different directions to spread like a plague.