Alright, figured out some things about Quixel Megascans.
Firstly - it's fucking expensive, way more than base software if you really want a decent library, and not depending on one type of leaf, two types of tree bark and a dozen types of wet dirt and mud.
After buying base you'd have to buy each scan pack separately and they're a big cost for little content.
Second - it supports creating your own assets, which could be useful.
Now the way I've "figured out" (because any moron would think of this) is getting a relatively good camera, going to places and taking pictures of different kinds of floor, also saving some leaves and rocks and shit in a container, then using photogrammetry to make them 3D.
After that, import into Megascans and use it as a compositor (seeing it's a damn sight easier to use it for this, even though you're the one getting the sources).
And that's the way to cost-effectively use this crap, assuming you're pirating the base itself, you'd just have to invest in a camera and time to go places.