The sudden drop in weight after even 1 day fasting is mostly due to water loss. If you do exercise and sweat a lot, you can expect 2~3kg weight loss

Calorie deficit is not really hard. Do not reduce too sudden and too much. Example: If you eat 10kg of rice, then reduce to 9kg per day. Maintain that for 1 week. You fill find you get used to it and do not feel hungry anymore. After that reduce another kg... You get what I mean.

Like people who fast during fasting month. The first day, they might be hungry but after 3~4 days, your body adjust to it. Your stomach will not secrete stomach acid to digest food at lunch time anymore. Your body will self adjust. Yes, no one die

Many people said they don't eat much. Well, the amount much is ambiguous. People in countries with not enough food are never fat

However, human body is clever. Once you start reducing food intake, your body also reduce its metabolism. So, at one pint you will find that your body weight is maintained.