If multiple routes match a path pattern, how can I tell which one would be used?
Your question is not clear enough.
Again, Your quote had full answer, the result is sorted by the fitness from highest to lowest fit. In case, they were equal, it would be sorted by alphabet of route name.
Return to your question, people usually use this function when they have a full or nearly full route name to check. or they have to add another condition that out of scope of this function.
You can not ask how Drupal determine which one wins when they are equal.