A video will describe your app better than a thousand words.
In the App Store you can have three videos max (aka App Preview). You need to have separate videos for iPhone, iPhone X and iPad, so App Store would show the most appropriate one to a user.
In Google Play a video will be displayed on the top of an app page, without autoplay.
Visitors should click the Play button to start the video. Here you upload only one video from YouTube, so try to to use landscape videos. Google has no time restrictions regarding video. In Google Play, Promo Video shows instead of Feature Graphics.