iphone 15 pro a3101 no instructions included

It's not a "problem", it's a feature. No iPhones come with printed manuals any longer (haven't for years); it's online.

To set it up, just power it on; it will tell you what to do.

Edit: most people never read them and they just ended up in a landfill.

There is an iPhone User Guide published by Apple if you care to take a look...

iPhone User Guide - Apple Support

Click on Table of Contents and read to your heart's content.