How do I turn OFF two-factor authentication?

Unfortunately, this cannot be disabled again, with one exception:

Source, Apple Support:

If you're already using two-factor authentication with your Apple ID, you can't turn it off. If you updated to two-factor authentication inadvertently, you can turn it off within two weeks of enrollment. If you do, your account is less secure and you can't use features that require a higher level of security.

You can find more information about Two Factor Authentication here

Two-factor authentication for Apple ID - Apple Support