The ammonia is just a chemical element meant to strip the layer of old fats off the hair externally.
The trick with the aged urine is this-
You're treating the scalp with lymph fluid. Urine is part lymph fluid, stuff that was drained off the blood which started subcutaneously, was carried out, and filtered into the urine. It is an easily-absorbed source of body-native nutrition..
And the aging process adds probiotics and digestive metabolites into the skin. The bacterium are breaking down urea, (oxygen, carbon, nitrogen, hydrogen) and producing enzymes, metabolites such as histamine and water-soluable nutrients (very small amounts of b-vits).
When histamine gets to the subcutaneous layer of the scalp, it draws bloodflow and immune cells to the area. The nutrients from the aged urine stay in the intercellular spaces and act (potentiate) new cell growth in the subcutaneous layer by providing the basic building blocks.
It's pretty much the same way wound-healing works in any injury on the body- blood wells to the surface, carrying white, red, and stem-cells-
*the white cells recognize and break down any foreign invaders/dead cells and remove the dead matter (eats it)
*the red blood cells regulate oxygen supply
*stem cells initiate new growth, sampling the types of cells existing around them and growing into similar cells.
So, in conclusion, aged urine has histamine and nutrients in it. The histamine kicks bloodflow and an immune response into gear where it's applied, and the nutrients act as a growth medium. The bloodsupply to the hair gives the scalp and the hair roots the chance to regenerate.
As a probiotic, a bacterium which can survive in urine is a potential help to the skin. Skin has a consistent colony of microorganisims which in part act as barriers to pathogenic species, though the quality of the skin (the fatty layer and amount of bloodsupply) has a major role to play.