Drupal Create Bootstrap 5 Subtheme

Below steps are tested with Drupal 9 and bootstrap 5.

Step 2: Click on Appearance

Step 3: Click on installed Bootstrap 5 theme Settings

Step 4: Navigate to Subtheme section and set subtheme location, subtheme name and subtheme machine name

Step 5: Click on Create

Your subtheme is ready, check in Appearence section.

To override page.html.twig copy from location templates/layout folder and create similar directory structure.