môžeš si prečítať https://en.wikipedia.org/wiki/Tree_shaking a https://developer.mozilla.org/en-US/docs/Glossary/Tree_shaking
nástroje používam niekoľko, na JS napr.
https://webpack.js.org/guides/tree-shaking/ a https://developers.google.com/closure/compiler/
pre CSS k webpacku čtení: https://survivejs.com/webpack/styling/eliminating-unused-css/