To make videos responsive, mostly anything with <iframe> use the below function, and don’t forget to add the css as well.

Make <iframe> videos responsive.php

// Add responsive container to embed videos
function forty_responsive_video( $html ) {
  //add http protocol
  $html = str_replace('<iframe src="//', '<iframe src="http://', $html);
  return '<div class="flex-video">' . $html . '</div>';
}
add_filter( 'embed_oembed_html', 'forty_responsive_video', 10, 3 );
add_filter( 'video_embed_html', 'forty_responsive_video' );

Styles for .flex-video.css

// Make videos responsive
.flex-video {
  padding-top: 0;
  padding-bottom: 56.3%;
  position: relative;
  
  iframe {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
  }
}