Userscripts are a way to add extra functionality to a site using Javascript.
To use them, add either the Violentmonkey or Tampermonkey extension to your browser.
https://violentmonkey.github.io/
https://www.tampermonkey.net/

> Where To Get Userscripts

GitHub Gists: https://gist.github.com/search?q=%3D%3DUserScript%3D%3D (Search for UserScript)
Greasy Fork: https://greasyfork.org/en

> Userscript Development Resources

How To Write A Userscript: https://simply-how.com/enhance-and-fine-tune-any-web-page-the-complete-user-scripts-guide
GM_* API: https://violentmonkey.github.io/api/gm/