[fastball]

I'd assume all the graphics computation happens in C++ code via WebAssembly, which is then rendered in the browser via WebGL. The Typescript part is the glue and all the non-gfx parts of the interface, like the top bar / sidebars / etc.

Since Figma is also all about multiplayer, I imagine they might have a system that takes changes to a document, packages them up in a compact binary format, and then sends that over the wire (to Figma or to other connected clients). A decent decent target for a WASM module would probably be that serialization/deserialization step.