Direct from the horse's mouth - watch time and the contribution your video makes to viewer session time. On the Seo side, titles and the first few sentences of the description. Things like CC and intl translations help foreign viewers as well. Playlists help organize your content into viewer friendly lists. Tags are way down the list. Things like backlinks, external embedding, are pretty much useless unless they contribute to watch and session time. Anything that kills the viewer session should be avoided like the plague, ie don't take viewers off Yt to social media.