How do I get a downloadable USER GUIDE

To explore the iPhone User Guide,You can ➔ download the guide from Apple Books (where available).

Axel F.

Open the Books app, go to the Books Store and search for iOS 15 user manual and iPadOS 15 user manual (they may be combined) and add them to your books library.

A long time ago, Apple used to have PDF versions of the manuals available. But no longer. As the others have replied, you now have to use the Books app.