Great question, Audrey. The “top highlight” isn’t set by the writer. It’s determined by whatever the readers highlight after a story is published.
For example, the top highlight I’m seeing right now for this story is the part screenshotted below that was highlighted by three different people.
As the writer of this story, I have no control over the highlighting. Readers highlight whatever jumps out to them, and Medium lets future readers know which part most resonated with the previous readers of that story.
Let me know if that makes sense or if you have any follow-up questions!