Code
Here, various SVG components are implemented to form an interactive background. This includes such elements as <path>, <line>, <ellipse>, and <g>.
UI
On the homepage of Envy Labs, the background shapes react to the position of the mouse cursor. This is likely set up with a combination of JavaScript (JS), Scalable Vector Graphics (SVG), Cascading Style Sheets (CSS), and HyperText Markup Language (HTML).
UX
The interactive background enhances the User Experience (UX). It provides visual effects that many users would likely consider visually satisfying.
Summary
Overall, Envy Labs has done a great job with their homepage. The Developer Experience (DX) could be improved somewhat, but, other than that, there are no significant problems.