If you didn't purchase your phone from an actual Apple Store or actual Apple Authorized Seller, there really isn't anything you can do. There's nothing we can to help you. Without a receipt for the phone from Apple directly or an Apple Authorized Seller, Apple won't help you. So you go buy a new phone if that's your situation.
Here is a link for you to see if whoever you purchased the phone from is an Actual Authorized Seller --> Find Locations- Apple
If whoever you bought the phone from isn't listed, you're really out of luck. I'm sorry this happened to you, but you let it happen, so please learn from this experience and NEVER EVER let someone sign into your device to begin with.
A final note: The scammers will likely send you a ransom demand to let you back in your phone. From reading what other's who have made this unfortunate decision, once they pay, the scammers just demand more money and never let you back in your phone.