Daz has a handful of basic primitives built in (spheres, cubes, cylinders etc) but it can also import models in .OBJ format, which is great because that's about as close to a standard that the 3D world has available. (You can also export models in .OBJ format too for editing in other software or 3D printing)
The inner workings of 3D rendering engines are way beyond my level of understanding, but in general I believe they work on a layered surface manner rather than genuine raytracing, although you can still do raytracing like effects such as transparency, reflections, specularity and so on. Not too sure if it handles caustics properly though, as I just tend to work with the default stuff rather than anything that advanced - I'm sure there are plent of far more talented artists in here who could tell you much more than I ever could though.
The basic figure models (Genesis, Genesis 3, Genesis 8) are built in to the basic software and there are literally terabytes of add-on content that you can install, all available on the internet, though while Daz is totally free, most of the good content has to be paid for (unless you happen to know of a few slightly dodgy Russian websites where you can find 90% of it free of charge, if you get my drift
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In general, using Daz is really very simple once you understand how the basic tools work, and there are tons of very good tutorials on Youtube that will help you get started. The one I started out with was this series, and by following this guy, you will learn it pretty fast, and best of all, for most of the first few videos in the series, you only need the assets that come with the program so you won't need to install any other stuff;