No video Rosa Theme

Hello everyone!

I am new to wordpress, and, also a bit of a n00b when it comes to PHP. Inevitably, I installed the plugin, added the video, but it creates a featured image but no video is displayed. Even the [featured-video-plus]-Shortcode shows up as a small video, no where near like a background video. And I see in some threads there is mention of adding “the_post_video( $size )
has_post_video( $post_id )”. But I have no idea, how, where, and what should be edited with those lines of codes. Could somebody help me please? This is for the latestest Rosa Theme.

Thank you.

https://ww.wp.xz.cn/plugins/featured-video-plus/