I know you can use the app for it, but I’ve never done so. No reason to download somthing extra if I do not need to

You don’t need the app, at least not for the mp3 player part, just plug it in to a pc and drag and drop into the location

This is however only the case for watches that support mp3s like my venu sq music

My old Garmin watch doubles as a mp3 player, is quite nice for running since I don’t have to carry anything