YouTube videos can be displayed fairly reliably in apps. However, only horizontal (landscape) display is allowed. Video prepared in vertical (portrait) mode, is limited to the height of an already tiny landscape mode display:
This is the case even though YouTube now supports vertical video.
You may be thinking “But only an idiot would make a video in portrait mode!” Au contraire . . .
From an engagement perspective, there have been a number of interesting studies that found merit in using vertical video. Breadnbeyond, for example, found that smartphone users hold their phones vertically about 94% of the time. Why create content exclusively for the other 6%, rather than capitalizing on the 94% that’s readily available?
By the way, though horizontal YouTube video is supported it has been my experience that mp4 support is very limited in AppSheet. See the following discussion: