I am newbie in Drupal. Using some articles, I have created a child theme of Bartik in Drupal 8 but I do not know how to override default template files. Can anyone help me to understand the child theme structure in Drupal 8?
In your child theme's folder create a subfolder called "templates". In this folder you should store your own templates. Just copy the system's (or parent theme's) template to your folder and modify it.
Further on, you can create more special templates, i.e. not only node.html.twig but also node--blog.html.twig.
Finally, switch on twig debugging, then you will get comments in your html like this:
<!-- THEME DEBUG --> <!-- THEME HOOK: 'block' --> <!-- FILE NAME SUGGESTIONS: * block--bootasde-branding.html.twig x block--system-branding-block.html.twig * block--system.html.twig * block.html.twig --> <!-- BEGIN OUTPUT from 'themes/bootstrap/templates/block/block--system-branding-block.html.twig' -->
Then you always now which template was finally taken.