How to unlock an iphone 11 pro max when showed lock to owner?

You won't like this but if you see a "Locked To Owner" message, one referring to iCloud locked, or activation locked, and you don't have the sign in information, you're out of luck.

Take it back to the person from whom you received it. Get them to do the things in this support article or ask for a refund --> What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support There is no way to discover the owner of a device unless they left a message to display on the device when it is turned on, or the person who gave you the device knows them.

From the very bottom of Apple Trade In - Apple :

"Does Apple offer recycling?

Yes. Apple Trade In lets you recycle any Apple device (including devices from Apple-owned brands) at any Apple Store and on Apple for free. That includes your batteries and old electronic products as well as free, on-demand packaging recycling for our commercial, education, and institutional customers. When we receive your device, it will be thoroughly inspected to determine if components can be recycled or reused. Whether recycled or reused, all activities relating to the processing of your device will be managed in an environmentally responsible way."

If the phone is not yours then you are not able to unlock it.

If the phone is yours, you can enter your Apple ID and password.

Which is it?