Legal Question in Personal Injury in California

Since there is no specific objective number for general damages, there is no way to testify to the actual number. So no one does. The injured party testifies to their own objective impacts on their life from the injury. Medical experts can testify to future medical treatments that may be necessary and the consequences. From there it is up to the plaintiff's attorney to argue for a number to the jury in closing argument.

who testifies to the general pain and suffering damage dollar amount?

The complaining plaintiff and witnesses as to the 'symptoms, effects, and consequences', and his financial experts to their opinion of 'value' going forward.

No mystery, not incredible, SOP