If you’re using standard html tags when you paste the video from YouTube into wherever you’re adding it (should just be the body of the text edit area?) you should be able to change the width to 100% (and then remove the height tag). Shouldnt need to use css. It looks like the embedded video has a fixed width that’s larger than the frame on mobile?
So it could be something like <video widthequals100%> ’url here’ <\video>
I’m not a coder tho so please someone with more knowledge chime in.
Edit. I’ve typed equals so the forum doesn’t think it’s actual html, so use an = sign instead of the word