What does Google count toward prominence?
Google names two things: links and reviews. Its definition, in full: "Prominence means how well-known a business is. Prominent places are more likely to show up in search results. This factor's also based on info like how many websites link to your business and how many reviews you have. More reviews and positive ratings can help your business's local ranking."
Two details are worth knowing. "Based on info like" means links and reviews are examples, not the whole list. And the summary line on the same Google page currently reads "relevance, distance, and popularity," while the section itself is headed prominence. Both words describe how known and talked about your business is.