Regarding the pH testing instructions, I understand that the first urine is supposed to be more acidic than the second, but I don't understand how the saliva test plays a role here. Why do we test it and what should it be like?

Saliva pH shows alkaline mineral reserves (thus digestion ability). Aiming for close to neutral (7). Urine naturally collects metabolic acids and acquired dietary acids (from the day's grazing) during sleep and releases in the morning. So the 2nd urination of the day is a better measure if the body is too acidic. A second pH that's closer to neutral (7) demonstrates good kidneys, and acid readings demonstrate that the microbiome and body would appreciate more fresh organic vegetables and fruit.