If you would like to make your embedded videos responsive to the screen size your viewers are watching the video on, the code to add is fairly simple, but it is based on the aspect ratio of the video. For YouTube videos, this information can be found under Share >> Embed code. For example, this What Is OER video has a embed code that indicates the video is 560 wide and 315 high has a 16 by 9 aspect ratio. Below is the code for responsive videos of the most common aspect ratios. Below is also an example of code for a video that has additional playing code for a video. To translate and width and height to an aspect ratio, use an aspect ratio calculator. 

<!-- 21:9 aspect ratio --> 
<div class="embed-responsive embed-responsive-21by9"> 
<iframe class="embed-responsive-item" src="..."></iframe>
</div> 

<!-- 16:9 aspect ratio --> 
<div class="embed-responsive embed-responsive-16by9"> 
<iframe class="embed-responsive-item" src="..."></iframe> 
</div> 

<!-- 4:3 aspect ratio --> 
<div class="embed-responsive embed-responsive-4by3"> 
<iframe class="embed-responsive-item" src="..."></iframe> 
</div> 

<!-- 1:1 aspect ratio --> 
<div class="embed-responsive embed-responsive-1by1"> 
<iframe class="embed-responsive-item" src="..."></iframe> 
</div>

Example

<div class="embed-responsive embed-responsive-16by9">
<iframe allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen="" class="embed-responsive-item" frameborder="0" src="https://www.youtube.com/embed/VjGflXu6FSk" title="YouTube video player"></iframe>
</div>