Steven Universe maybe doesn't have the best worldbuilding, although I think it's sufficient to sustain the show. It helps to think of it something like a tokusatsu, in which world-shattering invaders that threaten all of humanity and Earth nevertheless are polite enough to confine their attacks and manifestations to a particular Japanese city. The show gives us terrifying Gem-monsters able to smash up city blocks—so, are these things to be found all over the world? I guess the Crystal Gems can warp around the globe if they want, but there's no clear indication that they're attempting to monitor the entire world for Gem-monster incursions. Mostly they stick close to Beach City.
But it's such a great premise. Long ago, on some alternate Earth, there was an alien war, a war so destructive that it altered the geography of the planet, and ever since there's been the shattered remains of an alien occupation of Earth, scattered bits and pieces of Gem technology and Gem soldiers. Strange interactions occur between some of these Gem relics and the substance of Earth, and thus we get oddities like the flowering moss from "Lars and the Cool Kids" (illustrated above)—possibly some sort of accidental fusion between a terrestrial plant and Gem fragments that happily resulted in the creation of a new lifeform.
(Wouldn't human beings try experimenting with Gem fragments themselves? The implications of that are frankly terrifying—especially if we make the assumption that the human beings of Steven Universe Earth as are foolhardy and ambitious as human beings tend to be here on our Earth—so maybe it's just as well that the show didn't really go there.)
The fact that Gem relics create this sort of weirdness around themselves, causing the creation of novelties and monstrosities of various sorts, has caused me to speculate that one could stitch together the continuities of Steven Universe and Undertale. The origins of the Monsters in Undertale is completely unknown and I doubt whether Fox or anyone else on the development team gave the matter much deep thought; all we really know is that they're magical beings. I've entertained the speculation that the Monsters are creations (deliberate or accidental) of human mages—but I've also got this wilder idea that the Monsters' origin is tied up with the aftermath of the Gem occupation of Earth. The interaction of Gem material with earthly things produces results that might be regarded as magic, after all.
(I do have some more particular ideas about how it might work but, uh, just wanted to run that up the flagpole.)
~Chara of Pnictogen
