How to add HMTL code in the child theme

Hi there,

you don't add HTML in a child theme - well not directly, it would have to be within a PHP function that outputs the HTML.
This article explains adding a video background to a Header Element:

https://docs.generatepress.com/article/page-hero-background-video/

The HTML goes inside the Header Elements text area.
The CSS for it can go in your Child Themes styles.css