A great many Indian sweets begin with milk boiled down for hours in a wide pan. The technique is laborious and it is not replaceable, because two separate things happen during that time.
What reduction concentrates
Milk is largely water, with protein, sugar, fat and minerals dispersed through it. Boiling removes the water and leaves everything else behind in an increasingly dense mixture.
As the proportion of solids climbs, the liquid moves through stages a cook can recognise by feel: pouring, then thick, then grainy, then a mass that leaves the pan.
Each of those stages is the starting material for a different sweet, which is why one pot of reducing milk can lead to several unrelated results.
Why browning happens alongside
Milk contains both protein and its own sugar, and the two react with each other under sustained heat. The mixture takes on colour and a distinctly cooked, nutty aroma.
That reaction needs time as much as temperature, so it develops gradually across the hours rather than arriving at a single moment.
It is the reason reduced milk tastes nothing like fresh milk with water removed, and why powdered milk reconstituted as a shortcut tastes flatly of dairy.
Why the pan is wide and shallow
Evaporation happens at the surface, so a wide pan removes water far faster than a tall one holding the same volume. The traditional vessel is broad and heavy for exactly this reason.
Heaviness matters because milk solids settle and scorch easily, and a thin pan develops hot spots that burn the layer sitting against it.
Constant scraping of the sides returns the drying film to the pot, which contributes both texture and colour to the finished mass.
Where acid enters the process
A second family of sweets takes the opposite route, splitting milk deliberately with acid to separate curds from whey. That gives the soft fresh cheese behind the eastern sweets.
Here the goal is a tender curd, so the milk is split quickly and drained rather than cooked down. Overheating at this stage toughens the protein permanently.
The two traditions produce completely different textures from the same starting liquid, distinguished only by whether the water is driven off or drained away.
Why sugar goes in late
Sugar added early slows evaporation and raises the risk of scorching, since it caramelises and sticks well before the milk has finished reducing.
Adding it near the end lets the cook judge sweetness against a mixture that is already close to its final concentration.
Cardamom, saffron and nuts follow for a similar reason, since their aromatics would largely cook away across a long reduction.