In my opinion, a glass of milk right before bedtime will not help much: tryptophan per glass of milk is about 120 mg, which is quite a bit. And melatonin takes time to develop from this tryptophan. A full stomach is not the best way to sleep.
The opinion that warm milk helps to fall asleep did not arise from scratch. The fact is that milk contains an amino acid - tryptophan, which has sedative properties, as well as calcium, which helps the brain to absorb tryptophan. In addition, warm milk causes a slight increase in body temperature, which in turn relaxes and soothes you.
Plus there is a placebo effect. We unconsciously associate warm milk with childhood. There are pleasant feelings of calmness and drowsiness. And you literally fall asleep like a baby.
Other drinks can also help, for example
I have never tried it personally. I will refer to the effect of a glass of warm milk with cookies at night, which is described in a very famous film. The film is called "Roman Holiday".