youtube background video without plugin

Or use a plugin? It’s the exact same code with or without a plugin.

https://ww.wp.xz.cn/plugins/search.php?q=youtube+background

The reason I suggest using a plugin is that that way your feature will be easier for you to support.

Or locate a theme that does that as a theme feature. I am not aware of any theme that does that in the WordPress repo. 😉