Google Play preview videos are YouTube videos attached to the store listing. They are optional for most apps, but Google highly recommends them for games and uses them in several discovery surfaces.
The upload model
You add one preview video by entering a YouTube URL in the preview video field. Google says to use a video URL, not a playlist or channel URL, and not to add extra parameters such as timecodes.
Visibility rules
The video should be public or unlisted, not private. It should not be age restricted, and it needs to be embeddable on Google Play.
Localization
Google recommends localized video tailored for different markets. If the video uses on-screen copy, localize the UI, taglines, and audio where practical.
Source notes
Google's preview asset docs describe the YouTube URL field, visibility requirements, localized video recommendation, and copy restrictions for preview videos.