how to unlock my iphone 11 pro max

Unlock what? Carrier Unlock or Activation Unlock?

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 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 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.

What exactly happened to the iPhone?