core-js is the more common of the two in our index: it runs on 17,396 websites against 7,940 for Swiper, and 4,011 sites use both. Counts come from real scans, not vendor claims.
core-js is a modular standard library for JavaScript, with polyfills for cutting-edge ECMAScript features.
Swiper is a JavaScript library that creates modern touch sliders with hardware-accelerated transitions.
4,011 sites run both core-js and Swiper. That is 51% of the sites using Swiper, the smaller of the two.
Want to know which one a particular site uses? Look it up and see its whole stack, or compare two sites side by side.