Any questions that can be automatically graded by Moodle will be graded when the student submits the quiz. If all of the items in your quiz are able to be automatically graded, then no action is needed from you - the student grades will be automatically transferred to the gradebook.
If you have one or more manually-graded question types in your quiz, use the following steps to assign marks to those questions and give written feedback.
Once grading is complete for a quiz, the student will see their result in the gradebook.
Additional helpful information on building quizzes can be found here on Moodle.org: Building Quizzes.