Drupal is a cmf developed by Dries Buytaert about 20 years ago. It has developed a rich culture of developers supporting the system, and it is a highly flexible system.
When developing a website it will make the process much easier if you can develop the whole stack on your local PC. This page describes how to get started.
Composer is a way to organize different parts of a project in code.
When building a website you can have a project which is your website which imports other components such as modules and themes.