As Wildo6882 said, you can use Google Play Music on both an Android Phone & an iPhone. If you buy music from Google Play on your Galaxy S6, it will automatically add it to Google Play Music, and so it will show up on your iPhone. Even if you don't pay for the streaming service, it will still sync music you've purchased and any music you've uploaded from your computer (you can upload your entire iTunes library* so that you can stream it from the cloud, rather than store it on the phone's physical storage).
I use Google Play Music and it syncs seamlessly between my old android phone, my iphone and my Mac (in the browser). I buy all my music from Google Play and have no issues playing it on my iPhone.
*Doesn't work with DRM protected songs.