Fabricators are what you get when you take a really fancy 3D printer and stick an AI art program inside.
Yinrih have a form of machine learning algorithm known in Commonthroat as <sfBqbcg> [yip, early falling weakening whine, huff short low strengthening whine, short low weak growl], or “leasemind”. (“lease” is used here in the archaic sense of “false”). While it's probably used elsewhere, the most frequent use for leaseminds is in fabricators.
You can give a fabricator general commands and it will try and build what you ask for. However, fabricators usually have preloaded schematics with exact instructions on how to build something, and the leasemind is just there to troubleshoot small problems that arise during the building process.
The Dewfall is equipped with a fabricator, and its leasemind is used more extensively than usual. The missionaries try to adapt their technology to human ergonomics by feeding the leasemind data on humans and asking it to make normal yinrih tools adapted for human use. Just like real AI art programs, it often messes up. This usually takes the form of assuming humans have various yinrih features, like a muzzle, whiskers, a tail, extra thumbs, or prehensile feet.
The fabricator can't just replicate matter, you have to keep it topped up with raw materials (think metal powders that are melted and molded, silicon ingots cut into wafers, petroleum for making plastic, and so on.)
Edit:
A hasty edit to address some pretty glaring economic implications: general purpose fabricators like you see on the Dewfall, for the most part, can only make commodities. More specialized products require more specialized fabricators. I'd be happy for anyone to poke more holes in the idea, or perhaps suggest ways this would effect how the economy works.
Let's assume a fabricator can produce a keyer and HUD specs, which are a pretty ubiquitous form of portable computer. This would imply that, when you needed a keyer and HUD specs, you wouldn't go out and buy them directly. You'd instead buy the raw materials needed to make them. If you wanted a new model of keyer and HUD specs, you'd purchase a schematic license like you purchase a software license in addition to the raw materials.
hmmm… Writing this down has forced me to consider the ramifications of such a device. At the end of the day what I need from an external perspective is a way for the missionaries on Earth to be able to produce a mass router without needing to know exactly how a mass router works. The missionaries aren't geniuses. This is probably Stormlight's wheelhouse, but he's not going to know how to turn raw aluminum and silicon into a mass router, although he's skilled enough to understand the principles of operation once they're explained to him and could operate one with some coaching.