These services act as intermediaries. They have access to global databases (often through inside connections at manufacturers or carriers) where they can retrieve your unique unlock code based on your IMEI.
Advanced users sometimes use SP Flash Tool with a scatter file to flash a patched modem firmware. If you flash the wrong secro.img or nvram partition, you can erase your phone’s IMEI or cause a “no service” error. This requires professional repair. Nokia Ta 1283 Unlock Tool