Nothing goes out on a new Series until you've seen it. Your first post is always held for review, whichever way you set the Series up, so you can check the mood is right before the rest follow. It waits on the series page: press Approve & start schedule to send it out and start the rhythm, or Skip this post to drop that one.
After that, posts follow the Before each post publishes setting you chose when you created the Series. Auto-publish sends every post to YouTube on schedule. Manual approval holds each one for you, to approve, re-roll or skip. You can't change that setting on the series page later, so set the Series up the way you want it to run. A connected agent can change it for you.
Later posts on manual approval wait in your Library. Open Library → Generations, press Publish on the post, and confirm in the publish window. If you want to change something first, press Open editor and re-roll the visual or the music there. Re-rolls use usage like any other render.
Approve a post before its slot and it goes out at the slot. Approve it after the slot has passed and it goes out shortly after you approve.
If a post seems stuck waiting, Why a scheduled post was skipped has what to do. Anything else, reply to this or email [email protected] and a human will read it.