How to use my iPhone 6s Plus?

See if anything here helps:

iPhone User Guide – Apple Support (AU)

That iPhone is quite old. I was not able to locate a specific User Manual for the iPhone 6s Plus in Apple Support. But you can use Google or another search engine to look for help. I noticed several instructional YouTube videos available during my search.

We’d be happy to try to help if you have specific questions.

Sometimes those include linked 'Table of Contents' with a bit more.

iPhone User Guide – Table of Contents: Apple Support (AU)

//support.apple.com/en-au/guide/iphone/toc

[Used to be more regional TOC versions; somehow those don't appear.

Listed iPhone models are there, they include 'supported models' now.]

In apple books, older manuals used to be either online or download.