SPFx and Bootstrap

Add the following to the config.json Example below is using bootstrap 3.x but you can use any version

"externals": {
    "jquery": {
      "path": "https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js",
      "globalName": "jquery"
    },
    "bootstrap": {
      "path": "https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js",
      "globalName": "bootstrap",
      "globalDependencies": ["jquery"]
    }
  },
  
  Import Statements
  
import { SPComponentLoader } from '@microsoft/sp-loader';
import 'jquery';
require('bootstrap');

Loading the CSS from CDN and start using Bootstrap. 

let cssURL = "https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css";
SPComponentLoader.loadCss(cssURL);

Happy Coding