Confirming they run ads on many videos. Some are more bearable than others, and I'm really not sure if it's an uploader choice, a network choice or what.
Some videos have only one small mobile-like rectangular small add located at bottom-middle of the video, it tends to appear about 15-30 seconds in.
Other videos feature a series of such things, popping up every couple minutes or so, that's bloody annoying.
Other videos yet have a full-screen ad autoplaying right after the regular video ends.
And finally I've seen a couple occasions where the ad starts before the regular video starts, full-screen, unskippable, bloody fucking annoying. I usually just close that video and if this happens repeatedly for the same channel, I unsubscribe from that channel.