What is the process of rain water harvesting in residencial houses ?

Answer 1

Reclaimation.

Answer 2

OK, let's say you are not in the tropics, you divert your downspout from the gutter (kits are available at DIY shops), and collect your rainwater in a plastic drum which must have a lid (I actually use 2 "wheely bins" because a rainwater drum in a garden center costs $100.- and the garbage bin was $19.--.
It then depends what you want to do with that water, if you want to use it for watering plants or topping-up a fishpond, use it as is. But for flushing toilet, or even being consumed, it has to be processed accordingly.

Answer 3

in tropical climates where they have little fresh water, they dig a basement and put a tank in it to hold water, then build the house over it, and the roof is covered with a gravel type material and painted with lime, the lime sort of sanitizes the water that will go into that tank from the gutters instead of out onto the ground.