When someone ask "On which technology is based Facebook/Instagram/Google?", never answer "PHP", or "Python", or "C++". Truth is that web-projects are built on spread of technologies.
They are so complex in implementation, as difficult it was to understand. If there are no analogues, if you can't to spy on something, if there is nothing to copy-paste, you have to invent it yourself.
Initial requirements was to build XML RPC service for UI based on AngularJS to allow search, recolor, resize and download icons in SVG and PNG format. My work on this project started with test task, which become a main part of api.icons8.com.
I worked with Icons8 for 3 years. Nowdays it is a greatest icons collection made by single design studio.