Grading one paper at a time is how you calibrate. Once you are confident, you can grade the whole class in a single pass and then review, instead of opening each paper yourself.
Unlocking it
Bulk grading opens once you have crossed the calibration trust bar, that is, confirmed enough papers yourself that the assistant has learned your standard. See Calibrating grading to your standards. Until then, you grade and confirm papers individually.
Running it
Select the submissions and choose Bulk grade. The whole set is graded together in one run: every paper is graded at the same time, so a full stack finishes much faster than opening and grading each one on its own. You can watch its progress, and stop it if you need to.
You still decide
Bulk grading proposes scores for the whole class at once, but it does not release anything. When it finishes, you review the results, adjust anything that needs it, and only then release. Grading at scale and releasing are still separate steps, so nothing reaches a student until you say so.