New phone apple id troubles

So I’ve got a new phone and have moved everything over to the new one but when I tried to download snapchat it came up with me having to put in my girlfriends apple id. Now alittle backstory, my gf thought it might be a good idea to put her iTunes on my phone so I could have her very large collection of songs that I’d probably never listen to on my own. When we found out it didn’t work I took the account off of my phone and even as I check the emails attached to this phone it’s only mine so why do I need her password for a download.