How do I turn off two-step verification on my iPhone?

Did you really mean the old security method, "two step"? If yes, read the article: Two-step verification for Apple ID; click here --> Two-step verification for Apple ID - Apple Support

See the section "How do I turn off two-step verification?

If instead you mean the current two factor authentication, two-factor authentication is enabled by default on new Apple IDs and cannot be turned off. If you have an old account pre-dating Two Factor Authentication and only recently activated Two Factor Authentication, note this section of an Apple support article:

"Two-factor authentication for Apple ID" --> Two-factor authentication for Apple ID - Apple Support

--> "Can I turn off two-factor authentication after I’ve turned it on?

... If you recently updated your account, you can unenroll within two weeks of enrollment. Just open your enrollment confirmation email and click the link to return to your previous security settings ..."

Presuming that you meant to write, “Two Factor Authentication” …

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First: 2-Factor Authentication (2FA) protects your AppleID, not any individual device.

Second: It can’t be turned off.

So … learn how to use it by carefully reviewing these two articles:

Two-factor authentication for Apple ID - Apple Support

Get a verification code and sign in with two-factor authentication - Apple Support

Setting-up additional Trusted Numbers - in advance - becomes particularly important in the event that your device is broken/lost/stolen.