I'm not involved with figma in any way, just a user. But from what I understand, it is written in c++ and uses webassembly. Some ui parts are in standard TS+css. [1]

There is absolutely no reason not to be responsive, low on memory and as powerful as a native app.

[1] https://www.figma.com/blog/webassembly-cut-figmas-load-time-...