How do I add external CSS OR JS libraries in Drupal 8 ? 

You need to add external library (CSS, JS) into your_theme.libraries.yml and attach the library either in twig or in your controller


myLibrary:
js:
https://xyz.com/bootstrap/4.1.11/js/bootstrap.min.js: { type: external, minified: true }
css:
component:
https://xyz.com/bootstrap/4.1.11/css/bootstrap.min.css: { type: external, minified: true }

{{ attach_library ('mythemename/mylibrary') }}


What is TWIG ? 
Drupal uses a new theme engine called Twig. Its a flexible, fast, and secure PHP based theme engine.its templates are written in a syntax that’s less complex than a PHP template and easier to create interactive and more functional Drupal templates using Twig.

What is Clean URLs ? 
user-friendly URLs, or search engine-friendly URLs, are Uniform Resource Locators (URLs) intended to improve the usability and accessibility of a website is called Clean URLs.
In Drupal 8, ‘Clean URLs’ are enabled by default and can’t be disabled. Check your site’s URLs which should not contain ?q= within the URL. It has certain dependencies on Webserver modules to work properly.

Which are all the Symfony components used by Drupal 8 ?
Symfony Components used by Drupal 8 project :
ClassLoader
Console
CssSelector
Debug
DependencyInjection
EventDispatcher
HttpFoundation
HttpKernel
PHPUnit Bridge
Polyfill Iconv
Process
Routing
Serializer
Translation
Validator
Yaml

how to improve DRUPAL performance for anonymous and authenticated users ?
Anonymous Traffic
Lighter user, basic configuration and performance tuning should handle more use cases.

Authenticated Traffic
Heavier user, can lead to more complex configurations to handle load and performance.
Caching for Drupal Websites

so here are five of the most important caching solutions for a Drupal website:

Drupal’s built-in caching
Boost module
Memcached
Varnish
Views caching