Put a URL in a post and the composer fetches the page behind it and shows you the card that will go out with it.
The URL stays in the body, because that is what every network builds the card from. Take it out and the card goes with it. Attach an image or a video instead and the card disappears, which is what the networks do too.
Each network draws it differently
X puts the title in a pill over a full-width image. LinkedIn stacks the image over a tinted strip with the hostname under it. Bluesky is closer to LinkedIn but keeps its own hostname row. The chip in each account's column is that account's version, so a link that looks right on one and wrong on another is something you find out before you publish rather than after.
Pages with no image get the compact card instead of a hero.