Can someone here explain how this works? I'm not sure how reliable it is, but here's[1] an old Reddit comment describing the method that Desmos (2D) uses. That method works in 3D, so maybe Desmos 3D uses it, too. [1]: https://www.reddit.com/r/desmos/comments/qlhmbc
After playing a bit with it, it looks like some variant of the marching cubes algorithm, and it looks recursive, as expected. Same as your Reddit thread but in 3D. There are most likely a bunch of optimizations, but I can't think of another way of doing it. There are raytracing/raymarching techniques too, but these look different, and they are most likely cost prohibitive for arbitrary function. There is the marching…
It's @iquilezles's work who is a ray marching wizard, in the most literal sense of the word that still comports with reality.