The homing location is wherever the limit switches are, and you can tell GRBL to reverse one or more of the axis directions to get it there.
GRBL, when configured for CNC use, assumes that the switches will be rear-right, and then you’ll manually zero the machine to the workpiece.
There’s a compile-time flag that you can set when building GRBL to tell it otherwise:
// After homing, Grbl will set by default the entire machine space into negative space, as is typical
// for professional CNC machines, regardless of where the limit switches are located. Uncomment this
// define to force Grbl to always set the machine origin at the homed location despite switch orientation.
// #define HOMING_FORCE_SET_ORIGIN // Uncomment to enable.
It’s an annoying hold-over from CNC use.