Normally, YouTube generates source for embedded YouTube video like following:

<iframe
  src="https://www.youtube.com/embed/33pe6pnw9C8?controls=0"
  title="YouTube video player"
  frameborder="0"
  allow="accelerometer;fautoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture"
  allowfullscreen></iframe>

In this theme, when iframe element has src attribute that starts with https://www.youtube.com/, it is recognized as embedded YouTube video and width and height is adjusted automatically.