You can start by finding a few good music download apps for iPhone, although most of them would require a paid subscription to actually save the music offline (but the streaming can be free). These include apps like Spotify, Apple Music, YouTube Music, SoundCloud, etc.
When you want to transfer a YouTube video to MP3 format, chances are you’re looking to save an audio track. We get it; tons of YouTube videos have awesome music or soundtracks, so why wouldn’t you want to save them?!