Making a video embed responsive requires a bit of custom code.  Just put the “embed-container” around your embed code:

<div class="embed-container">
  <iframe width="1150" height="700" src="https://www.youtube.com/embed/y6V-5Uai57Y?rel=0&amp;showinfo=0"></iframe>
</div>

This page has a working example: https://www.getrefm.com/refm-software/apartment-building-development-back-envelope-analysis-calculator/