How do you backup iphone 11 to windows?

There is still iTunes on Windows and you can still backup on a Mac with Catalina that replaced iTunes with separate apps with similar functionality. Instructions for both are in How to back up your iPhone, iPad, and iPod touch - Apple Support

How to get iTunes:

Update to the latest version of iTunes - Apple Support

Backing up, etc:

Back up your iOS or iPadOS device in iTunes on PC - Apple Support

Locate backups of your iPhone, iPad, and iPod touch - Apple Support

You need to have iTunes on the computer to do a backup. See here for help: https://support.apple.com/en-us/HT203977

duggram1 wrote:

There is no iTunes, so what do you use?

Well, I’ll take a wild guess this means you have a Mac running MacOS Catalina. If so, you may still back up to it.

How to back up your iPhone, iPad, and iPod touch - Apple Support