Right on the page you linked is this line:
The game is quite volatile so the RTP ranges from 90.93-95.17% dependent on choice of free spins.
that's all you need to know right there.
That's just a number, I what to know how is the number derived.
It's very complicated.
1. The probability of bonus occurrence in various patterns is combined and then calculated RTP
2. The probability and bonus of free game
3. Finally, calculate the benefit according to the probability of jackpot
The total return is 85-93%, as different casino Settings affect the size of the player's bet.