At which point does a delta 3d printer change the G-code in order to work in its non-cartesian design?
The short answer is that the handling of the non-cartesian design is done by the motion-control firmware running on the Arduino.

The long answer:

I don't believe GRBL supports non-cartesian designs, and it is not commonly used for printers. It is more often used for mills, routers, or laser machines. 3D printers will typically use a firmware such as Marlin, which supports several printer designs, including Delta machines.

At no point is the g-code itself changed. The motion control firmware running on the Arduino or other controller interprets the g-code and determines which way and when to step each motor to accomplish the motion.

With a simple cartesian machine, commands for the X-axis only relate to the X-axis motor, but for a non-cartesian machine the axis and motors have complex relationships. The firmware must be programmed and configured to control the motors correctly.

The g-code itself is never passed to the drivers. The commands to the driver are simple electrical signals to "enable" (to energize the motor power - even to just hold position), "direction" (which way to rotate the motor shaft), and "step" (which causes the motor to rotate by one step in the selected direction).