Below steps are tested with Drupal 9 and bootstrap 5.
Step 1: Install Bootstrap 5 theme
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.