Generically, it would be referred to as "OTP NVM", or one time programmable non-volatile memory.

There are apparently a few different ways to implement that.

A little dry, but the only thing I could find with some comparison of approaches: http://www.kilopass.com/wp-content/uploads/2010/04/compariso...

AFAIK it's unknown - there's a controller on the SoC which is responsible for reading/writing the fuses, but its firmware is in mask ROM and isn't available. Perhaps it's a real chemical reaction? Maybe it's just emulated in firmware? Your guess is as good as mine.