You can download songs from a torrented website, extract it and move it to your iPhone via iTunes. If you have some extra cash, it's better to use Apple Music imo.
Get the app from the big multi-national coffee house retailer. They regularly feature free songs and app downloads from the iTunes Store. I personally prefer copy left or open source musicians and artists who offer their music through Creative Commons. There is a wealth of priceless music out there. You just got to look.