Switching Apple Accounts or What is ON My Phone?

I’m going to spare the huge preamble about why I need this information, I just need it !

There is a iPhone 5s sitting next to a 6s Plus. The Apple account used to initially set up the 5s is for all intents lost (password not remembered, e-mail not accessible). Folks here helped me figure out how to deal with the contacts, but I JUST found out there is something like gigabytes of music involved. I doubt that iCloud on the 5s was ever enabled or set up. My question is how does one KNOW exactly where the music and the contacts reside? I THINK some of the music was purchased through the store, but it was more than 6+ years ago and I THINK at that time, it DID get physically downloaded to the phone.

If the data IS ON the phone, then I should be able to swap the old phone to a new account, set up iCloud, have it back it u, then move to the new phone.