bootstrap4.css
.embed-responsive .embed-responsive-item,
.embed-responsive iframe,
.embed-responsive embed,
.embed-responsive object,
.embed-responsive video {
  position: absolute;
  top: 0;
  bottom: 0;
  left: 0;
  width: 100%;
  height: 100%;
  border: 0;
}
■ディスプレー:Display:ユーティリティ:Embedユーティリティ 埋め込みクラス アスペクト比 16:9

  <div class="embed-responsive embed-responsive-16by9">
    <iframe class="embed-responsive-item" src="https://www.youtube.com/embed/zpOULjyy-n8?rel=0" allowfullscreen></iframe>
  </div>