I think Onana's mistake was he made a slightly late step to his left, Salah hits it first time and even if Onana guessed it was a pass to Salah he would still have to make a guess on when Salah will shoot, and which side he will aim his shot at, Onana most likely in that second thought Salah will bend it to the other side hence him angling his body to his right and his late step to the left, it was a quick attack and he was backing to his goal line since he was away from it.
Szobzlai before passing to Salah, notice Onana is focused on making sure he keeps an on Szobzlai in case he takes a shot
When the ball is passed to Salah, Onana is trying to adjust in get central
When Salah takes the shot, Onana was in a spot to allow to react in case Salah aims at the far post
What could have been done better by Onana is perhaps to guess that the ball will go to Salah and readjust his position earlier but it is not an egregious error for me.
I still believe he is not a good GK in terms of saving shots or making good decisions when facing attacks, first goal he should have moved better and faster to his right side, second goal it was literally right under him and he should've saved it, third goal I think it is bit too hard to deal with.
Edit: Compare Onana to Allison, when Zirkzee shot, you could see Allison already guessing that Zirkzee will aim at the far post and Allison was able to save the shot, wasn't an easy one to save but because Allison is an excellent GK, he can make such saves, Onana most likely does not.