I have an Android with a microSD card that I save all my music to. Just copy it from my computer, and slip the SD into the phone, and I have all my music.
I am sure that you are shocked to hear that iPhones can’t use those small, convenient SD cards for internal storage.
I do have a memory stick that uses the lightning adapter but its not what I am looking for.
Searching thru Apple support docs seems to point in the direction of my using a DRM-less encoding solution.
Way back in the 80’s I worked on a piece of software that would play back MP3s on the TI 99/4a. Discovered that Forth and Assembler could certainly do the trick, if I was patient enough. I wasn’t.