How to turn off two-factor authentication on ios 11.1

I don’t recall having to enter a 6 digit number?

Then why do you think it is enabled?

In order to set 2FA up, you have to create a trusted device. That means sending a verification code in real time to your phone and proving to Apple you have control of that device.

I don't recall activating two-factor authentication and I don't want it activated as I don't use icloud.

If during setup of your phone, you provided a phone number and then entered a 6 digit code that was sent to that phone, you enabled 2FA.

It is used for any system that accesses your Apple ID, not just iCloud.

If you create a new Apple ID these days, you will be required to use 2FA. I don't believe it's an option to opt out.

Based on this...

Two-factor authentication for Apple ID

Availability of two-factor authentication for Apple ID - Apple Support

You can try to turn it off here in the Security section.

Apple ID - Manage

Ok.