There are passenger baggage cargo interline agreements which are more extensive than the usual alliance and co-op partners. So they can actually have your bags tagged through on separate tickets. I flew Air NZ to Singapore then Qatar onwards. After I sent my baggage through I got talking to one of the agents and they were like.... yeah we could have tagged your bags through SIN and onto QR so I didn't need to pick it up in SIN. Would have been nice but not a big deal as they had some 24hr check in at SIN for a bunch of airlines and I'd left loads of time between flights.