I forgot my iPhone's password

Password or passcode? A phone uses a passcode.

We need to distinguish between a passCODE which is a four or six digit code used to unlock a mobile device after you turn it on, and a passWORD which is what is used to log into your Apple ID account. I am not clear to which you are referring.

Read the Apple Support document about what to do if you forgot the passCODE on an iPhone or iPad. Click here then select your device type if necessary --> If you see an iPhone or iPad unavailable message or Security Lockout screen - Apple Support - Which one you can use depends upon your equipment configuration.

Click on this link to read what to do if you forgot your Apple ID passWORD - If you forgot your Apple ID password - Apple Support

or go directly to this link --> http://iforgot.apple.com/

If you still need help:

Contact Apple for help with Apple ID account security "This article provides country-specific Apple Support contact information for customers seeking help with their Apple ID password or other security-related issues." - Contact Apple for help with Apple ID account security - Apple Support

Password or Passcode?

To unlock your iPhone, follow the steps described in the Apple article:

(< iOS 15.2) ->: If you forgot your iPhone passcode -> If you forgot your iPhone passcode - Apple Support

See -> Change your Apple ID password -> https://support.apple.com/100193

Then see this Apple article: What to do after you change your Apple ID or password

What to do after you change your Apple ID or password - Apple Support

BTW:

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