And when people "root their phone" they are just modifying part of the device. The baseband is closed source and illegal/impossible to modify in order to protect the network and spectrum.
Some of us do get pretty close and modify stuff like EFS to enable/disable functions (voNR) and enable bands that were present but not legal at the time the device was certified. In US, band 77 was disabled on many devices but later became legal to use. The manufacturers didn't want to pay for the recertification but the device is capable otherwise. We also sometimes add band combinations (for carrier aggregation) that the manufacturer missed.
