The main concepts while managing quiz content (questions) are:
You can think of a quiz, in essence, to be like a traditional pen&paper quiz (or an exam/test). It contains questions. With Moodle Quiz, you can also give the grading beforehand for the questions in a quiz (as well as the total for the entire quiz).
When you create questions, they are stored in the Question Bank. In the Question Bank you can create categories, which are similar to folders. You can use them to create a hierarchy for organizing questions, for example, by topic. When you create a question into an exam, a copy of it is stored in the Question Bank. If you decide to remove your question from an exam, it will still be intact in the question bank until you delete it from there, too.
You can use Random Questions, if you want a question to vary between different attempts students make at the quiz (for example, to avoid cheating by means of students copying questions to each other). Just create a random question in the quiz and add questions in the category of the random question.