I think that it is something anyone can do, provided that they know how. I would assume that it is as simple as putting the phone into some kind of low-level programming mode, perhaps EDL or similar, and overwriting a few bytes of data.

These low-level modes likely use proprietary protocols that are only known to the vendors who manufacture the SoC used in the phone, for example Qualcomm. As many of those devices are manufactured in China, it is likely that someone who worked there leaked some tools for performing these low-level programming activities. And then they figured out how to use these tools for unlocking the bootloaders.