We have been doing SIM unlocks for many years. First, we used to buy iPhones and other smartphones in US and bring them to EU for resale, outside and abroad United Kingdom. Phones were usually locked.
There were many ways to unlock them. Back in the old days it used to be possible to unlock iPhones by updating firmware. Many people used to change their iPhone 3GS baseband to an iPad's one (the famous 06.15.00) but this came with a costly downside, the GPS stopped working.
Always existed a non-factory unlock method for other brands and models. Non-iPhone smartphones were mostly unlocked with a "box", where you use to connect your phone to a "magic box", pay some credits, and get your phone unlocked. Either way, it was not an official unlock, so it would void our phones warranty. That was unacceptable for high end phones (low end devices didn't worth the work bringing them overseas).
A simple solution for a big problem: there is a third way of removing SIM locks, unlocking by IMEI! It is also known as "factory unlocking", it leaves the phone as it had never been locked to any carrier, so no problem with warranty and happy customers.
We started unlocking the phones we imported and use them with any network. But soon, our customers -and also, our providers- started asking us to unlock their devices. After a few years, we were handling so many orders that it became obvious that our main business had switched to SIM unlocking!