Need to unlock my iPhone so I can use it on a different carrier

Who is your current carrier? Whoever they may be, you need to contact them and ask them to unlock the phone. Depending on your original agreement with the carrier, they may or may not be under any obligation to unlock the phone.

Only the carrier can unlock the phone. Apple is not involved with this at all.

How to unlock your iPhone for use with a different carrier - Apple Support

Note that the Related Article you were looking it is about Activation Lock – a different thing. Activation Lock is an anti-theft device designed to make lost and stolen iPhones worthless to thieves, fences, and their customers.

You'll need to contact your current carrier if the device is carrier locked.