Three.js
A free JavaScript library that lets a web browser draw and animate 3D scenes.
Three.js is the piece of software that makes 3D possible on an ordinary web page. It handles the tedious parts, talking to your graphics card, positioning cameras and lights, working out what is in front of what, so that a developer can describe a scene in fairly plain code and have it appear in a browser tab. It is free, open source, and has been the default choice for browser 3D for years.
It shows up constantly in AI demos for a practical reason. A model asked to build a 3D scene can produce a single self-contained HTML file that loads Three.js and runs immediately, with nothing to install and nothing to download. That makes results easy to share and easy for anyone to judge for themselves.