Modify YouTube Embed Code to fit theme

If you’re using the block editor, the fix is simple:

add_theme_support( 'responsive-embeds' );

You can also make the iframe responsive using pure CSS, just wrap it in a div:

<div class="yt-container">

</div>

    .yt-container {
      position:relative;
      padding-bottom:56.25%;
      padding-top:30px;
      height:0;
      overflow:hidden;
    }
    .yt-container iframe, .yt-container object, .yt-container embed {
      position:absolute;
      top:0;
      left:0;
      width:100%;
      height:100%;
    }

This article covers making embeds responsive using CSS and JS:

https://css-tricks.com/fluid-width-video/