I believe this happens inside a liquid substrate that cures (hardens) when exposed to light. Instead of building up a shape by exposing a series of flat layers (stacked on top of eachother) one at a time, this exposes the entire 3d shape at once, using holograms.

that was my first reaction

For lazy folks like me https://www.nature.com/articles/s41586-026-10114-5/figures/5