Im trying to find the User Guide for iPhone 13 pro Max. Can't seem to find it on Apple.com

There is no model specific user guide. Their are guides for iOS which cover all iPhones capable of running a specific version of iOS.

This is the iPhone user guide for iOS 15 -> iPhone User Guide - Apple Support and it will cover the iPhone 13 Pro Max

It is also available as free download in the iBook store.

Click on the table of contents and it will show all iPhone models that are covered by that iOS guide.

There is no iPhone 15. The iPhone user guides are organized by iOS version not by phone models. iOS 15 is the latest iOS version and the guide for it will note any features that don't apply to a specific model. One way to view the guide is to go to the following and click on Table of Contents: iPhone User Guide - Apple Support