how do you unlock an iPhone?

“Unlock” has many meanings:

• Remove the carrier (SIM) lock: You must contact the carrier the phone is locked to
• Phone is disabled and can’t be unlocked—>If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support
• The phone is Activation Locked to an Apple ID—>iCloud: Find My iPhone ACTIVATION Lock
• SIM PIN lock—>Contact your carrier

Carrier Lock:- Though your iPhone’s original Carrier is not obligatory to unlock your iPhone, but you may know the process here --> How to unlock your iPhone for use with a different carrier ...