Stock index futures, like other futures, all the funds in futures are transferred from some participants to another, so what they earn is the loss of others. (excluding handling fee)

If there are many more people who buy short than those who buy more, it shows that people have high expectations of price decline, so people who buy short will pay the bill by constantly lowering their declared price, so the price will fall until the price reaches the long-short balance. Simple relationship between supply and demand.