I was also facing this problem with many of my websites where i use videos. These days many videos are as reels i.e. vertical or portrait.

I tried many option like mentioned above. They work but for the sake of simplicity i created a plugin to make things simple. For people who may not be very comfortable with css or inline code.

This plugin is free and on wordpress. https://wordpress.org/plugins/yt-portrait-video-embed/

Simply use shortcode and video ID. And its done. I have tried to make it responsive as well. Hope its helpful to you.