core-js is the more common of the two in our index: it runs on 20,003 websites against 71 for lite-youtube-embed, and 27 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.
The lite-youtube-embed technique renders the YouTube video inside the IFRAME tag only when the play button in clicked thus improving the core web vitals score of your website.
27 sites run both core-js and lite-youtube-embed. That is 38% of the sites using lite-youtube-embed, 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.