How Do You Network Unlock a Phone?

If you go to a normal plaza or mall, there obviously would be that one sketchy tech/electronic store that offers repairs/phone unlocks. How do those stores know how to network unlock phones? I'ts so weird that there isn't a way on how to network unlock a phone on YouTube or a forum post, and a random guy down the street opens up a phone repair shop and knows how to unlock phones! How do they do it? Do they pay a company to get access to the tool they use or what? I really want to know how to unlock phones without paying a website online or physically paying someone. Please tell me because maybe one day I'd like to be that guy opening an electronic shop down the street.

Thanks,

-Brandon