Unlocking iphones

How is it locked?

• Device disabled message —>If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support
• SIM locked to a carrier: Contact the carrier it is locked to and ask about their unlocking policy—>How to unlock your iPhone for use with a different carrier - Apple Support
• SIM PIN lock—>Contact your carrier for the SIM PUK code
• Apple ID locked—>If your Apple ID is locked or disabled - Apple Support
• Device Activation Locked—>Turn off Activation Lock - Apple Support. If it is activation locked you will need to contact the person who enabled it. If you can’t find that person return the phone; it will be useless to you.

You can't. There is no way to bypass activation lock without the owners Apple ID and Password used to set up the phone. If you don't who the user was and can't get the credentials, you have purchased a useless device. Return it for refund.

See this link --> The All Too Common SAD Reality of Buying … - Apple Community