Where can I find a iPhone 13 user manual?

AFAIK, Apple does not provide a specific user manual for each of their iPhones. Instead, they do provide user guides for the iPhone's iOS operating system and repair.

Online Version: iPhone User Guide - Apple Support

Repair Guides: Apple - Support - Manuals

The user guide is referred to the latest iOS version, now iOS 15.6, for all iPhone models supporting that version.

Click here:--> iPhone User Guide – Apple Support

Regards

Giulio

Try this one: Turn iPhone on or off - Apple Support

You're very welcome!