A youtube video could be added as Remote Video media, or embedded in WYSIWYG.
Various modules enable different display options.
Media can have extra display modes, embedded content can have different parameters.
How you should configure drupal depends on whether you want to reuse the embed, and available options for display.
A media type is the most robust, supporting reference fields and CKEditor embed.
https://codepen.io/mortona42/pen/mybqqqK
Here's the embed code, with height: 800px
See the Pen Untitled by Andrew Morton (@mortona42) on CodePen.
Here it is with max-width: 600px on the wrapper.
See the Pen Mantra Link by Andrew Morton (@mortona42) on CodePen.
This is a youtube video.
For max width, set the aspect ratio on the container, and iframe to 100% height/width.
Here's an older fix, that still works with backwards compatibility: https://www.w3schools.com/howto/howto_css_responsive_iframes.asp