This was actually easiest for me during the end of the dumbphone era. All the devices around then had Bluetooth and supported the Object Push Profile, and there wasn't yet any DRM implemented on sending stuff like MP3 files. It was slow but worked.

Then Apple killed Bluetooth by never supporting it well on the iPhone, and then implemented their proprietary AirDrop (which is actually LESS reliable for me, between Apple devices(!), than Bluetooth OPP was)