I'm transitioning from an iPhone 8 Plus to an iPhone 13, and had wiped the older phone ( having remembered to turn off Find My... and also log out from iCloud ).
All was good, and the new iPhone 13 is oh-so-shiny; I'm a very happy camper ...
However, as I'm handing down the older model to a family member, I wanted to go get the battery replaced.
The Apple site asks, perhaps quite wisely, for a serial or an International Mobile Equipment Identity (IMEI) number.
Try getting that from the Settings page on a wiped, yet-to-be-setup iPhone ...
Thankfully, this article had the answer: -
It's on the SIM tray; just grab a handy paperclip, and you're good to go !