Note that there are actually two code samples: one for transparent videos in android and another for transparent videos in iOS. 

• Android transparent videos have to be in "webm" format

• iOS transparent videos are recommended to be in "mov" format

• In both cases you have to enable alpha transparency and export the video with alpha enabled

Video links:

• https://kitcross.net/hevc-web-video-alpha-channel/

• https://developers.google.com/web/updates/2013/07/Alpha-transparency-in-Chrome-video

• https://blog.gskinner.com/archives/2021/10/alpha-video-in-html5.html