What does "it" mean here?
"It" is not actually referring to anything here. "That's not even the half of it" is an informal expression meaning that there's even more to the situation (good or bad). In this case, the speaker has used this expression to emphasize that the cellphones have even more amazing features than what has already been mentioned. Ex: That's not even the half of it. After I got fired, my car broke down and I had to walk home. Ex: That's not even the half of it! After I got accepted to my top university choice, I found a hundred dollar bill on the ground.