How do you turn off two-factor authentication to phone number

The 6-digit 2FA "special popup" is sent to all devices - not phone numbers - signed-on with the same Apple ID.

It's part of iOS / iPadOS / MacOS and can't be disabled.

Only if you setup and then explicitly select to use one of your "trusted numbers" is the code sent as an SMS text to a "phone number."

See MUCH more detail here:

Two-factor authentication for Apple ID - Apple Support

Ok, gotcha.

You can manage your trusted numbers from your Apple iOS device under Settings -> AppleID -> Passwords & Security.

You can also manage them via the web at https://appleid.apple.com/