iPhones unlocking: how do they do it?

I'm really curious through what kind of process these people who are unlocking iPhones are going through?

I'm also interested on how can they check the iPhone lock status through IMEI?

Does anyone have any related info, because it seems that I can't trust in google to find that kind of info, unfortunately.

And I'm really curious as I'm looking to buy 2 iPhones for my mother and my sister, but I don't really want to pay somewhat 100$ for each unlock.

 

you don't need to pay 100$ to unlock an iPhone. You can do it yourself if your firmware is compatible or bring it to a local store and they can do it for 30$ max. It's basically entering a code to bypass the "lock" on the carrier's network so you can use it with another one. You can buy iPhones from Apple which is factory unlocked by default.