Once in a quiz, Questions can be created directly inside the quiz by clicking "Edit Quiz" or directly in the question bank by clicking on "Question Bank" and then on "Categories" (in the admin block). Questions can also be made off-line and imported. Many docs are available on this topic.
If you don't intend for teachers to "share" quiz questions, I'd highly recommend that teachers create their questions at the "quiz" level instead of the default course level. This helps with long term organization (so you don't end up with hundreds of questions at the course level). This method also allows you to duplicate quizzes which will then make copies of each question (instead of 'sharing' questions).
Also consider a Naming convention for the titles Of the question (which is helpful if a question gets saved or moved into the wrong place or if teachers want to copy a question from one quiz and add it into another-Such as from a practice quiz to a final exam). By having the name of the quiz as the start of the question title, This helps you know where the question initially existing. That's just tidbits for my schools use case, you I'm sure will have your own use case. But check out the Moodle docs!