TrueView in-stream ads and video discovery ads appear, respectively:
before videos and as clickable thumbnails
as clickable thumbnails and before videos
to the left and right of videos
at the top and bottom of videos
Explanation:
TrueView Video Discovery ads includes the following old formats:
- YouTube Promoted Videos overlay
- Promoted videos suggestions and related videos
TrueView in-stream videos can appear on YouTube watch pages and on video partner sites and apps in the Google Display Network.
Read more here: https://support.google.com/adwords/answer/2375464