An idli steamer looks like an odd stack of drilled plates in a tall pot. Every part of the design follows from what steam has to do to a fermented batter.
Steam has to reach every mold equally
Steam condenses on the first cold surface it touches, giving up its heat there. A design that lets it pool at the bottom cooks the lowest tray and starves the rest.
The perforated plates and the gaps between tiers create a path upward, so vapor moves past every mold rather than stalling under the first one it meets.
Trays are also offset when they stack, which stops a solid column of metal forming and keeps a channel open from the water surface to the lid.
The batter cooks by hydration, not by browning
A fermented rice and lentil batter is already full of gas from fermentation, and steam heat sets the structure around those bubbles without drying the surface.
Dry heat would crust the exterior before the center set, which is why idli is steamed and dosa, made from the same batter, is griddled instead.
The result is a food with no crust at all, which is unusual, and the vessel exists to make that possible in quantity.
Condensation is a design problem
Water condensing on the lid drips back onto the top tray, waterlogging those idli while the lower ones cook normally.
Traditional pots use a domed lid so drops run to the edges rather than the center, and many cooks wrap the lid in a cloth to absorb what forms.
Pressure cookers are often used without the weight for the same job, since sealing the vessel would build pressure the batter does not need and would compact its texture.
Water level and timing are linked
The base holds a shallow bath, deliberately below the lowest tray. Boiling it dry is the common failure, and touching the trays waterlogs the batter.
Because steam at atmospheric pressure holds a fixed temperature, the cook controls only time and steam volume, which is why recipes give a duration and not a setting.
Boiling hard does not cook faster either, since the extra vigor only converts water to vapor more quickly without raising the temperature the batter actually experiences.
Fitting one into an American kitchen
Indian groceries and online sellers stock stainless steamers sized for standard burners, and the same stacked-plate idea appears in inserts made for electric multicookers.
A wide stockpot with a rack and small oiled ramekins reproduces the principle if not the capacity, provided the pot is broad enough to let steam pass around them.
The one thing that does not substitute well is a bamboo steamer, since the porous walls let vapor escape and the batter sets slowly and unevenly as a result.