Beginning website design

Good day, please don’t throw tomatoes at me, I have a question. I have never created sites on Drupal before, I only know HTML and CSS. Thanks to your lessons, I created a site on a local server based on my Bootstrap theme. How should I proceed correctly to customize the theme for my needs? Am I right to understand that first I need to change regions and blocks, forming a site layout from them according to my needs, adding pages, etc. (or is everything added and changed in some specific HTML file)? How should I properly change styles? Should this be done in the style.css file of my theme or is it better to do it in other files? THANK YOU VERY MUCH FOR YOUR ANSWER