Unable to open iTunes on my iPhone 6s.

Perhaps this may find best reply if you roll-your-own from a Search?

https://duckduckgo.com/?t=ftsa&q=how+to+access+iTunes+with+iphone+iOS15+&ia=web

IF for use with Windows-based computer, and/or to backup files there

that's one of many possibilities; and above link has several of those.

And iTunes in both Mac (before later version macOS & PC/Win)

uses iTunes a in computer to save & restore the iPhone backups.

..Various answers can be found across several support articles online..

An example of results appears here- but odds against it -as correct one.

Use iTunes to sync your iPhone, iPad, or iPod with your computer - Apple Support

//support.apple.com/en-us/108347

However all those answers, depend on the depth of original Question.

So the mission of iTunes changes, at the point of intro to newer macOS.

iTunes - Apple (current website from Apple)