Almost all kitbash assets come as one big fbx or obj file. The entire file uploads as one into UnReal or Unity or other 3D software and then you make instances of them as they're individually named. For Poser/Daz this is somewhat inconvenient as they don't work that way but all Kitbash3D models are already UV mapped (and most have textures), you just match the texture with the material zone name. I import the original obj's into Maya and then export each individual asset out as solo obj and then bring them in as props.
I was actually toying with the idea of converting the Kitbash3d kits into corresponding prop sets for Poser with the materials already added so you could just unzip them into your runtime and they'd be ready to go but wasn't sure if there would be any interest in that.
ALSO Blender is excellent for converting stl's into objects, and 3D printable stl's are insanely popular right now and thousands are available for free and there are many low cost Kickstarters that reward with a ton of themed stls.