The question bank is generally a part of a course rather than any individual quiz but it can be used to feed individual quizzes from pools of questions within a category or categories.
Enter the course in question.
Expand the Actions Menu (Gear icon) from the top-right corner of the page.
Select the More… option from the very bottom of the menu.
Scroll down and you’ll find the Question Bank section.
Select the Questions option.
You can see what questions are contained in each category, one at a time from the Select a category drop-down menu.
Once in the desired category of questions you can create a new question for this category from within Moodle, one at a time.
You can also modify the contents of a question bank category by moving or deleting questions en mass.