Eye trackers are used as well for those who unfortunately cannot move anything but their eyes.
That's the case in very bad/late ALS cases.
This technology might really be life changing for the very advanced cases of ALS - they often lose the ability to vertically move their eyes, which makes it very very difficult to work with eye trackers.
Really hope they'll make something like this available to the public.
Depending on if they can move their neck much they might have a forehead dot to track movement for a mouse cursor and a tongue/lip device for clicking. Then it's just onscreen keyboards. Other devices I've heard of are cheek movement or even eye tracking.
In addition to the options other commenters have mentioned, it is possible to fully control a computer with dictation, although it can be very arduous.
The term quadriplegia does not always refer to 100% paralysis, and some have limited use of their hands.
Really hope they'll make something like this available to the public.