<div class="row">
    <div class="col-md-6">
        <!-- 16:9 aspect ratio -->
        <div class="ratio ratio-16x9 mb-3">
            <iframe src="https://www.youtube.com/embed/zpOULjyy-n8?rel=0" title="YouTube video" allowfullscreen></iframe>
        </div>
        </div><!-- end col -->
        <div class="col-md-6">
        <!-- 16:9 aspect ratio -->
        <div class="ratio ratio-16x9 mb-3">
            <iframe src="https://www.youtube.com/embed/zpOULjyy-n8?rel=0" title="YouTube video" allowfullscreen></iframe>
        </div>
        </div><!-- end col -->
        <div class="col-md-6">
        <!-- 4:3 aspect ratio -->
        <div class="ratio ratio-4x3 mb-3 mb-md-0">
            <iframe src="https://www.youtube.com/embed/zpOULjyy-n8?rel=0" title="YouTube video" allowfullscreen></iframe>
        </div>
        </div>
        <div class="col-md-6">
        <!-- 21:9 aspect ratio -->
        <div class="ratio ratio-21x9 mb-0">
            <iframe src="https://www.youtube.com/embed/zpOULjyy-n8?rel=0" title="YouTube video" allowfullscreen></iframe>
        </div>
    </div><!-- end col -->
</div><!-- end row -->