> They had a custom language giving them full flexibility to achieve making a product in the browser that put them ahead of everyone else

From the article it sounds like it was only used for the prototyping system so a single code base could run in the iOS client and the Web client.

I believe the main UI of Figma and what makes it so performant and magical to use is C++ https://www.figma.com/blog/webassembly-cut-figmas-load-time-...