What is the “find my phone” app tied to?

So I had my iphone stolen out of my hands a nights ago by a group of 7 guys. Amazingly the next day, after activating the lost mode, one of the thieves called me and offered to give me my phone back for a finders fee. However the Lock Screen code was changed on my phone. I had the erase the phone just to be able to use it again. So I’m trying to figure out if they wiped the phone first and the “find my phone” app still works after the phone has been hard reset OR if they saw me access my phone using my code and changed the code giving them access to all of my data…