What to do if I forget my Apple id

Check your web browser autofill entries, the Keychain, and other records, or use Apple's iForgot service, or contact their Account Security team, or if you're the unit's original owner and have encountered the activation lock, submit an unlock request as described in the Related Article above.

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I switched my phones and forgot my Apple ID

And . . . you are sign in here .  .  .  with your Apple ID & Password.

Your Apple ID is the personal account you use with all Apple products and services. It includes the email address and password you use to sign in, as well as the contact, payment, and security details that you use with Apple.

You can’t merge multiple Apple ID accounts.

What happens when you use the same Apple ID & Password that you use to sign in here?

See the following Apple articles

Sign in with your Apple ID -> Sign in with your Apple ID - Apple Support

Manage and use your Apple ID -> Manage and use your Apple ID – Apple Support (UK)