Add jQuery to a page with one script tag from a CDN. Use the slim build when you only need DOM and events to save bandwidth. Always verify it loaded by typing jQuery in the browser console.
Including jQuery
CDN (recommended)
<script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>
npm
npm install jquery
// import
import $ from 'jquery';
Verify it works
$(function() {
console.log('jQuery', $.fn.jquery, 'ready!');
});