Theme preprocess and suggestions for Drupal 8 Bootstrap sub theme
Preprocess hooks are a common way in Drupal to add or manipulate variables that you can use in your Twig templates files.
Suggestions hooks is a common way in Drupal 8 to extend the templates you can use for any part of your code.
Here is a snippet of this, copy this in your theme.theme file and replace
MY_D8_THEME by... your theme name!