Does Restoring Backup restore the IOS as well?

Hi,

if you have updated the last IOS version there aren't any problems.

A factory restore erases the information and settings on your iPhone, iPad or iPod and installs the latest version of iOS, iPadOS or iPod software. 

--- > https://support.apple.com/en-gb/HT201252

If you're restoring your iPhone, iPad or iPod touch backup from a computer that requires your device to have a newer version of iOS or iPadOS, you might see an error that your backup can't be restored because the software on your device is too old.

--- > https://support.apple.com/en-gb/HT204184

It will not go back in iOS.

It will not install an older iOS version.