Unlock iPhone

Unlock what? Carrier or Activation?

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 ...

If you are the owner of the iPhone or the owner has shared all information with you then to open Activation Lock:- If it is Activation locked, read here --> Turn off Activation Lock - Apple Support Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Learn how to remove a device from a previous owner's account.

If you are not the owner of the iPhone and do not have the Apple ID and Password then there is no way that you can get into this iPhone unless the original owner shares the Apple ID and password with you. If you are lucky you can get into this iPhone if they (owner) accidentally remove the device from their apple ID.

if it means the sim card needs to be unlocked then you type in the pin

Use a SIM PIN for your iPhone or iPad - Apple Support

if it is the phone which is sim locked to another carrier you needs to contact them about getting it unlocked

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