Thanks for clarifying! To keep the full structure of the image/video on both desktop and mobile (instead of it being cropped), you’ll want to adjust the CSS so the media scales down rather than covering the area.
Look for the section’s background settings or custom CSS, and try changing background-size: cover; to background-size: contain;.
Let me know your theme name and how your hero is set up (image background or inline media), and I can give you the exact fix.