holding Saliba. When Saliba jumps, the Scouser has his hand on Saliba's shoulder, pulling him down.

Saliba couldn't get a full jump and therefore can't quite get his head on the ball.

If White holding the 'keeper's hand is a foul, then why isn't that?

1'32" here:

https://www.arsenal.com/video/men?entryid=0_53mbgtjs