An advertiser wants to engage new users outside of YouTube and increase the reach of their campaigns. What tool should they use?

They should use Google video partners. This is a network of publisher sites and apps beyond YouTube where Google video ads can serve, so a video campaign can reach people while they consume video content elsewhere on the web. Opting in typically increases unique reach and often at a lower cost per thousand impressions than YouTube inventory alone, which is why it is the standard answer for extending a video campaign's audience. Two practical notes apply. Performance should be reviewed by network, since video partner inventory behaves differently from YouTube and may suit some objectives better than others, and the same brand suitability controls and placement exclusions should be applied there, so the extra reach does not come at the cost of where the brand appears.