Every iPhone since the 5S uses data encryption using the Secure Enclave hardware encryption system. When you erase a device it deletes the encryption keys so nothing is recoverable from the device in human readable form. All that can be extracted from the device’s storage chips would be encrypted gibberish.
But I also would never give my AppleID password to anyone, ever. You can always setup your devices yourself without any help from anyone in any store. It really is not difficult -> Set up your iPhone or iPad - Apple Support
Note that if you give your AppleID password to someone, then you’ve not compromised the encrypted and unrecoverable data on your erased old device. You’ve compromised your online data in iCloud associated with your AppleID.
Never share your AppleID (or any online service login ID password) with anyone. The minute you do so, your online account data is compromised.