It's very very simple too. I made one that that just randomly changed the location within a specified area years ago. To learn the API. It didn't work very well. I wanted to grab the users actual location and offset it randomly. So a user could choose to degrade the accuracy of apps tracking their location.

But once you've set it as at THE mock app you can't get your actual location to offset (maybe this has changed since.)

despite my attempts to make sure it ran correctly in the background it would always stop working after a while. That was the hard part at the time , making anything run in the background without the phone killing the process or eating the battery.

What was your use case for it in Pokémon Go?