two-factor authentication

These documents covering 2-factor authentication, along with the quoted information from each, should help you.

Two-factor authentication for Apple ID https://support.apple.com/en-us/HT204915See this section especially Set

up two-factor authentication for your Apple ID

Get a verification code and sign in with two-factor authentication https://support.apple.com/en-us/HT204974

“Whenever you sign in with your Apple ID on a new device or browser, you'll confirm your identity with your password plus a six-digit verification code. There are a few ways you can get a verification code. You can use the code displayed on your trusted device, get a text or phone call, or generate a code from your trusted device.” ...

“If you can’t sign in, reset your password, or receive verification codes, you can request account recovery to regain access to your account. Account recovery might take a few days or longer, depending on the specific account information you can provide to verify your identity.

Before beginning account recovery see:

If you didn't receive your verification or reset email https://support.apple.com/en-us/HT201455

“Not sure where that email was sent? Here are some tips to help you find it.”

The first code is automatically goes when you sign in with Apple ID and password with a device , you again click on resent the code a new code is generated .

The account is secured no one apart from knowing the credentials can enter the account .