Roll back iOS 18.1.1?

You don’t. Going back to a previous iOS version is not supported and never has been. You will need to do some troubleshooting to find out what’s happening. No one else is reporting that ALL their apps are not working. Yes, some third party apps may need to be updated before they function but unless you turned the feature off apps are updated automatically.

Try turning off your iPhone and restart it. That often solves glitches.

Restart your iPhone - Apple Support

You don’t. Downgrading is not supported. Why don’t you try troubleshooting the problem instead…