What is System Data Storage
System storage contains core system files that are necessary for your iPhone to function and also some temporary cache files. There isn't any easy way to delete these cache files. You can try deleting your Safari history & website data and restarting your phone to clean some of the cache files. Occasionally (I do it once a year), I make a backup of iPhone data and reset it. This might take some time to do but certainly deletes a lot of the cache files and make your phone storage as it would be out of box.