Drupal 8 custom twig templates

I've abandoned the CMS framework world some years ago when Drupal was to the version 6.x.x and WordPress was at the initial stage of development.

This is because I like to write my projects in pure PHP.

I've started some project where I will use WordPress for one and Drupal 8 for another, because this is the client request.

While I was starting the creation of the layout on Drupal, I was stuck to the new templating system adopted from the version 8.

I've some trouble with twig and his tag system.

I was reading the Drupal 8 documentation and with the .info.yml and .libraries.yml, and I didn't found difficulties to integrate the JS, CSS.

I've included Bootstrap 4, the theme I using is stark.

I need to figure out how to create my custom markup for the layout using bs4 classes. On WordPress, it's more quickly but with the Drupal theme structure I can't understand what kind of files are required and what are a plus.

Can anyone give to me a quick explanation about this?