Unfortunately, the provided solution is not the one needed. I suppose the question was, when you created a quiz and afterwards decide to use these questions in a question bank, how is this possible?
The solution only shows the regular guide on how to build a question bank and new questions in the question bank.
Does anyone know how to get existing questions from a quiz into a question bank?
I have the same question, which, alas, the provided "solution" does not seem to answer.
The process to create a question bank from an existing quiz involves exporting course content from the settings page and selecting the quiz to export. Then you import the file (preferably into a sandbox course) and create the question bank from it. After you have imported it as a question bank (preferably in the sandbox), then you can go to the course settings page and import the course content, choosing just the question bank you need.
There are numerous posts in the Community about the process, but few have step-by-step instructions. Here are a couple of places where I've written lengthy instructions for the process (realize that the second one with step-by-step instructions was written in 2017, so things may have changed).