Sometimes I want to use questions from an imported quiz on another quiz. Since I didn't create the question, it's not in my Question Bank. I've had to recreate the question manually in the other quiz. This is fine for the occasional question, but if I want several of the questions in other quizzes, I had to get creative. If I wanted to break the quiz into several smaller quizzes, I would import it several times and then delete questions from the new copies so each was left some of the questions. Today, I happened to notice that when I did this, my Unfiled Questions bank suddenly had more questions in it. When I investigated, I found that all the questions I had deleted showed up in the Unfiled Questions bank. I experimented. I imported a quiz, opened the copy to edit, and deleted the questions (or groups). All the questions (except Text (no question)) ended up in my Unfiled Questions bank.
Update. I returned to my questions banks at the end of the day and found hundreds of questions in my Unfiled Questions bank. When I finished my previous investigation, I deleted the quizzes I had been using to test my theories. All the questions from those quizzes found their way to the Unfiled Questions bank. I didn't notice it earlier, because it seems to take Canvas awhile to get them there in bulk like that.
So, to get imported quiz questions into your question banks: import the quiz, delete the quiz, wait for the questions to show up, then move them to an appropriately named bank.