What is the way to use a readymade double boiler with whistle?
You are supposed to fill enough water such that you can heat the milk well, but not fill it to the top - you need a bit of space for steam to build up so the whistle will work. Fill it to a height somewhere half between the milk height and whistle height, you don't need to be exact though. 

When the whistle blows, the water will be at full boil, so 100 Celsius. The steel of the inner vessel is quite efficient in conducting that heat, so the milk will be close to 100 Celsius, but not at 100 (that's why it doesn't boil over). I don't know the exact temperature and don't have tables for pasteurization times in this range, so in this case, you have to trust that the manufacturer has checked these tables, tested the product, and ensured that the milk is safe at the temperature reached in its pot.