You might have revealed answers for security questions , data of birth , rescue mail address and its password to them .
They have completely changed the credentials .
if you still owe the device check is your Apple ID signed in or they have removed your Apple ID ( signed out Apple ID and signed in with their Apple ID ) from an iPhone If you forgot your Apple ID - Apple Support
Do your Apple ID still exists on server https://iforgot.apple.com/appleid#!§ion=appleid
The Apple ID might be example1@gmail.com
You can convert the secondary authentication account into 2FA Manage two-factor authentication from iPhone - Apple Support