Are there any ways to increase iPhone 6S+ storage?

I've deleted just about everything I can on my 16 GB phone to clear up space for music. I already have an app to store all my photos so they don't take space up and I constantly clear out messages so they don't either.
I was wondering if there was something where I could add storage to my phone where I can keep more than the 16 GB worth of music right in my library so I don't use data to listen to it.
I've read about external storage devices for iPhones but I don't know if they act as a backup or if they actually add storage as well.

No not at all.

You can try a scandisk iexpand flash drive

Dr, Cleaner. Miracle app - we have used it for about 2 months, and love it.