I want to add specific information to certain pop ups that only pertain to a specific string in the data. However, I only know how to configure all data's pop-ups at once. How do I do this?
You might be able to do this with a hosted feature layer view.
In the view layer, configure the pop-up the way you want for those select features. Then set a filter so only those features show up (hopefully there's something specific in the attributes that you can use to distinguish them?) In the original layer in the map, you can set a filter for the opposite criteria so that the features in the view layer don't show up in the original layer. That way they won't appear twice in the map.
Hope that helps!