On rare occasions YouTube will play exceptionally long videos as ads. When YouTube Red came out I got multiple entire hours long shows as ads (as a "free preview!") I'm pretty sure Ive gotten one of the movies they put up for free viewing as an ad before.
Obviously you can skip after 5 seconds or whatever but they hope to catch someone playing stuff in the background. Probably to increase their crappy view count for those features to sell actual ads later.
They get some control but it depends on the content how exactly it works. I think for normal videos they get a say right from the start where the breaks are. But I know one guy who does YT and he live streams and has to clean up every VOD because they just randomly pepper ads all throughout.