This person didn't own this music in iTunes. They are using Apple Music, which is a subscription service that has a fluctuating catalog of music. Right now there are some 100 million songs.
Even with actually buying the music or whatever (and without DRM), it's interesting to consider that digital distribution means that things can be pulled from distribution from one moment to the next, without any warning. And the nature of digital distribution means that in that case there's no remaining stock still to be sold off which you could hunt down, and no second-hand market to speak of, either. So you're immediately down to either piracy, or else nothing…