A TrueView in-stream ad view is counted when a viewer:
watches the ad for 30 seconds
All of the above
Clicks on the ad
watches the entire ad
Explanation:
YouTube video view count will be incremented when the viewer watches 30 seconds of the video ad (or the duration if it’s shorter than 30 seconds) or engages with your video, whichever comes first. Video interactions include clicks to visit your website, call-to-action overlays (CTAs), cards, and companion banners. If view counts on YouTube are a concern, it’s a good idea to make videos at least 12 seconds long. YouTube analytics doesn’t track views less than 10 seconds.
Viewers can choose to skip the video ad after 5 seconds. If they choose not to skip the video ad, the YouTube video view count will be incremented when the viewer watches 30 seconds of the video ad (or the duration if it’s shorter than 30 seconds) or engages with your video, whichever comes first.
Read more here: https://support.google.com/displayspecs/answer/6055025?hl=en