TrueView in-stream ads and in-display 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
TrueView in-stream ads run on videos served on YouTube or on a collection of sites and apps in the Google Display Network (GDN). These ads may also run on YouTube videos that are embedded on other sites or apps. They can also appear on Android and iOS YouTube apps, m.youtube.com (on iPad and Android), and on connected TVs. Ads appear before video and as clickable thumbnails.
Read more here: https://support.google.com/adwords/answer/2375464