How does carrier locking work?

What part of an android device is responsible for carrier locking? I understand that custom ROMs can sometimes be used to bypass a carrier lock, but how might I go about making such a ROM from stock?

This is complicated somewhat by the fact that the actual device I have, an Orbic Speed RC400L, isn't technically an android device; it runs busybox/linux, but its bootloader, adbd, and cellular systems seem to be android-like. I'm happy to experiment if someone can provide resources for the general android case.