what happens if you forget your apple id and factory reset your phone with the account still on it but still have the phone number it's connected to

My niece has an iPhone X and she forgot the password and disabled the phone. I factory reset the phone but now its asking for the Apple ID that was on the phone. She can't remember the password for the account but still has the phone number that's on the account. I sent a code to that phone number and typed it in to confirm it but then it said it would send details about the password reset to that account. That didn't make any sense to me because we don't have access to the account that's why were reseting the password.