Answer 1

Drupal is written in PHP, the Drupal 8 core use some symfony frameworks components. Its also use some JavaScript frameworks and twig theming library for the front end.

Answer 2

Most content management systems are written in PHP but also Java, JavaScript (Node.js), Python, Ruby and ASP.NET are used for the backend.

Answer 3

make sure you understand php, and a form of sql (usually mySQL) for the backend, server administration if running your own server, if not at least the basics of using .htaccess, learn required modules for server, etc.
Front end, html, css, and Javascript (usually jquery library as well) all are important. after that depends on the CMS, ill warn you drupal in particular is really difficult unless you are VERY skilled in php, id recommend wordpress if a beginner.