loaded an app on my old phone to give away to niece, why did it load it on my new phone?

Got new phone, reset my old phone to give to my 8 year old niece who lives in another state. Loaded TikTok on old phone but it also loaded on my new phone with out my knowledge. On the old phone I used my Apple ID to get the TikTok app and signed out to remove my Apple ID information. I don’t want this phone old one linked to my new one!

why did that happen??? The old phone was erased reset as new. I don’t want to send her the phone till I make sure they don’t get none of my information and if they load app I don’t want it loading on my phone!