How can I download an iOS backup to my Windows 11 computer as a file?

Download from where?

to backup your iPhone to your windows PC, use the Apple Devices App. connect your iPhone via USB-Cable, click your device if not selected automatically, then click general. Trust your device and/or PC if necessary. There, you should be able to create a new backup stored locally on your windows PC. All information is found here: How to back up your iPhone, iPad, or iPod touch with Windows - Apple Support

You can't. Why do you even want to? You can backup your iPhone to iTunes, where you can then restore the backup to your iPhone from your computer using iTunes. But storing the backup as a file in Windows isn't supported. Windows is a completely different operating system from iOS.

Not sure what you actually want to do.

Are you asking how to copy/paste a previous backup file of your iPhone that was stored using iCloud over to a PC?

Are you asking how to copy/paste a previous backup of your iPhone that was made on a Mac over to a PC?

Something else? Please provide as many details as possible and assume that we know nothing about your current setup.