How the hell do Figma and Webflow work?

I saw my friend who’s a freelancer use these two today and I my jaw dropped 😳 They are so good and smooth. I didn’t know such complex, powerful tools can be built for the browser. Engineers that work at these places or similar - what’s the magic? Web flow looks like it’s all rendered on a canvas. Do you build your own rendering layer on Canvas, and then your own UI toolkit (like GTK?) on top of it? Is any of this open source? I’m so fascinated I want to jump ship 😂 #software #swe