Not apparent at first, but this is a TypeScript to c++ compiler at the core (https://github.com/geastack/compiler), with bindings for various platforms.

What do they use for graphics and audio?

Each platform has its native bindings. For graphics, you write CSS and HTML canvas API as well as JSX, it renders to native components and their alignments, the canvas API is converted to native surfaces, whatever the platform uses.

I mean, is it OpenGL, Vulkan, Metal? etc...

Ah this is for embedded apps. Pretty neat!

It also supports iOS, macOS, Android, Windows, Linux, XBox for Three.js games, as well as (some) Node.js apps.

"The earlier implementation could not be driven to zero of them, because boxing was load-bearing in its emitter"

In the docs of the repo of their TS to C++ compiler. https://github.com/geastack/compiler