CSS, Responsive, Video | Type: CSSEmbed Responsive YouTube Videos

First, the CSS:

.youtube-video { aspect-ratio: 16 / 9; width: 100%; }

Then add the class to the iframe embed code:

<iframe class="youtube-video" src="https://www.youtube.com/embed/xxxxxx" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>

Reference Links

https://yoast.com/how-to-make-youtube-videos-responsive/