Well, given that adult Maws seemingly exist to breed, eat, and present environmental hazards, it would probably begin by selecting a relatively flattish portion of the horizon, with preference for anything overbuilt such as to have living things on it.

And then it would destroy the lot of it, churning it all underground like a plow, to create a large, round, flat area of mostly-clear space. In the non-dream space, this would then become a nice spot to trap food and eventually lay eggs, but in the horizon it's probably just going to be completely wrecking any domain it finds itself in, like some kind of acid-spitting spaceworm kaiju.

But the thing about Thresher Maws, behavior-wise, is that once something is moving around in its territory, and it notices it, it doesn't stop focusing on that thing (and on trying to kill it). The only way to drop aggro is to leave, and even then, the bigger ones might still chase you a good ways.

So all Julie really has to do is show up, make enough noise that it notices her, and then fail to die when prompted, so that Team Thresher Maw can make the strongest possible surprise impression on its sleeping body.