This is a step by step guide for using a video as row's background. You can see the example of video background here.
1 - Open your desired page in "edit content" mode
2 - Click on the plus button in the bottom left corner of the screen and open the Element panel.
3 - Choose a row shortcode from the left sidebar
4 – Click on the "background button" at the top of the row.
5 – Choose “video” from the row background
6 – Here is the explanation for each option:
WebM file URL: add a link to a video file which ends with .webm
MP4 file URL: add a link to a video file which ends with .mp4
Video Preview Image: this image will be shown when the video is not playable for any reason.
Also, some mobile devices don’t support background video and the video preview image will be shown instead.
When you've added the video, click on "Apply" button. Then publish your changes using the Publish button on the top bar.
Choose a video link from your website host not from the other sites. It’s recommended to use both WebM and mp4 files. In case the user browser does not support one it will play the other format.