They take some window like 10 sec, 1 minute etc., measure volatility of returns of the assets they have on their platform, then use some weighing function (for example, weight for EURUSD = daily trading volume of EURUSD / total trading volume on platform) to calculate weighted average - aggregate volatility. This should be the index quoted on the platform. We can’t rule out that they add some random component to make it more difficult to decipher the formula. I guess predicting where the index price will go is very hard or even impossible.