That would be a later jailbreak, not a baseband unlock per se.
Geohot's first iPhone hack, IIRC, used an unsecured JTAG pinout he managed to find on the PCB that allowed direct write access to the firmware. Later it was discovered that the system bootloader had enough exploits that the process could be done entirely in software.
Later iPhone models would gradually ramp up security to the point that nothing of this sort could be done very easily even with physical access.
Geohot's first iPhone hack, IIRC, used an unsecured JTAG pinout he managed to find on the PCB that allowed direct write access to the firmware. Later it was discovered that the system bootloader had enough exploits that the process could be done entirely in software.
Later iPhone models would gradually ramp up security to the point that nothing of this sort could be done very easily even with physical access.