What if one page doesn't have field data, but others have?

Google Search Console is already grouping pages together. Core Web Vitals will be doing the same. If one (for example newly launched) page doesn't have sufficient amount of visitors, it might fall back to and re-use the data of similar pages. If Google could not find similar pages, it will fall back to the origin summary.

Currently, one can only see if there is sufficient data for a specific page and/or origin summary. You might need to look into Google Search Console to get an idea of page grouping.