The majority use Daz3D. It's a 3D program focused mainly around rendering characters. It requires a beefy computer due to long render times. A lot of stuff like characters, clothes and locations also has to be bought(unless you can make it yourself), but there is a fair amount of free stuff around as well, the issue is a lot of people use it as well so it might not be especially unique.
Another big one is Honey Select 1 & 2. It's a porn "game" made by Illusion Software. It's basically an erotic dollhouse if you will. Which makes it pretty useful for making naughty images. It's a bit easier to get into. But it won't produce nearly as nice results as Daz3D since it's all rendered in real time, being a video game. On the plus side, no need to spend ages rendering images. Just take a print-screen. If you want more of a pure anime art style, their other "game" Koikatsu is also pretty popular. There are a few other Japanese games out there that do similar things. But I don't remember their names. Custom Maid 3D?
If you are really brave, there's always Blender(free) or Maya(paid). They are proper full 3D modeling programs and require A LOT of knowledge and energy to use. Especially if you want to make photo-realistic stuff. But if you learn it, you can produce entirely unique content and even sell it, and has some excellent tools for making animations.
Some other less recommended stuff you can also check out includes: Source Filmmaker(pretty old at this point, but it was a big one for making porn images of video game characters), The Sims 4, Skyrim & Fallout 4. But I wouldn't recommend ever monetizing games made with them.