The graphics are sent to an attached monitor. From what I have been told, any CD player can perform this task with proper circuitry.
The key to achieving this is the "proper circuitry". This proper circuitry (a chip set somewhat like the audio chip set in the AppleCD SC) must tap the signal coming directly from the laser, not a downstream location such as the SCSI connector. The laser controller must also be aware of the need to scan the graphics tracks as well as audio tracks. Since the circuitry is needed simply to find the graphics tracks, the graphics information is NOT available within the AppleCD SC.
Adding the graphics mode to any CD player requires the initial design to allow that mode. Since the AppleCD SC was designed and manufactured prior to the CD+G format becoming a reality, adding this feature is not possible: a completely new design would be required to support the CD+G disc format.
The same situation faces support for CD+V, CD+MIDI, CD+MIDI+G, CDI, and CD-DVI; each different disc format requires supporting circuitry to decode the appropriate digital information recorded on the disc.