I hope you are well today and thank you for your question.
It seems you are referring to the below demo content that is created using the Unyson page builder.
You can set the background video to the section as shown in the attached screenshots.
The video autoplay does not work on mobile due to some restrictions as described on the below page.
For the same reason, the video does not work on smaller screen mobile so in this case, you can set the image to display on mobile as shown in the attached screenshot.