Which airlines do they search?
All of them. Like even Southwest, which is famous for not playing with online travel companies. It also scans smaller regional European carriers, and that can mean serious savings on connecting flights once you get across the pond.
Is this a European-based site or does it work here?
It works all over the world. In fact, Langes says that Sydney-Vancouver is actually one of the most popular routes they’ve sold, and that customers have also reported cheaper flights to Asia too. That said, don't expect dramatic savings on US domestic flights, Langes warns, as those fares are are already pretty low. It's still worth a search before your next trip though, to play it safe. Just maybe not 40 of them.