As it builds the collection, you should see a fair amount of logging about which movies are being added and which ones aren’t found. Once it completes, go to Plex, go to your Movies library, and click “Collections” at the top.
Each time you run the script, new movies that match the collection definitions will be added. For example, if you don’t have “The ShawShank Redemption” now, when you download it and run Kometa again it will be added to the IMDB 250 collection.