The process is known as electrolysis. It’s the same technology that the United States Navy uses to supply oxygen to the Mariners on the submarine.
I use an o2 grow and love it but I use it in conjunction with air stones.
They say you can actually dissolve too much oxygen into your water, not with an o2 grow but in general. I have tried to exceed those saturation points and never been able to do it lol, therefore I just add all the oxygen I can.
I have seen diy videos on YouTube about making them but have never tried myself.
For electrolysis to take place there needs to be salts in the water, this is the reason the submarines can derive oxygen from ocean water so it will not work in a situation like a flush where you are running 0ppms.
Also, the emitter needs to be cleaned every reservoir change because it builds up allot of gunk as a byproduct of electrolysis.