I think you are confusing activities with questions. Forum, Lesson, Quiz are types of activities, as are resources like Label and Page, etc. They belong to the course and appears in sections. When you delete a course, all activities in it are deleted.
The Question bank holds only questions. A question bank can can have question categories at the course level and also the activity level for any activities in the course that use questions, like Quiz. The question bank has access to higher level question categories located outside the course, too. (See https://docs.moodle.org/310/en/Question_bank for more.)
When you delete a course, it will delete all questions located at the course level categories, such as the "Default for course", and any of its subcategories you may have added. It will also delete any activity level questions live categories at the activity level in the course (if you have made any). Deleting a course does not delete questions that live at higher level question categories such as System or the Course Category levels.
Hope this clears things up.
The Question bank holds only questions. A question bank can can have question categories at the course level and also the activity level for any activities in the course that use questions, like Quiz. The question bank has access to higher level question categories located outside the course, too. (See https://docs.moodle.org/310/en/Question_bank for more.)
When you delete a course, it will delete all questions located at the course level categories, such as the "Default for course", and any of its subcategories you may have added. It will also delete any activity level questions live categories at the activity level in the course (if you have made any). Deleting a course does not delete questions that live at higher level question categories such as System or the Course Category levels.
Hope this clears things up.