How does carrier/network lock work? How is it implemented?

I feel like I should have been able to find the answer to this, maybe I'm distracted or something, but does anyone know how carrier/network/sim lock actually works and if there are any workarounds? Say I wanted to make one of those unlocking services. What is the process? I saw a couple answers from some years ago, they all seem fairly vague and as if it's actually an unknown, so I'm not sure if that's current information. It seems inevitable that someone would have leaked or reverse engineered that information by now, right?