I don't understand how this would work. If I sell a product on my website for 10$, that works on multiple platforms, would each platform be able to take 30%?

I don't understand how this would work. If I sell a product on my website for 10$, that works on multiple platforms, would each platform be able to take 30%? At that percent and 4 platforms this would be net negative, so that seems wrong... Conversely I could say the product costs 10$ for the PC platform (which does't have the commission), and you get a 'free' version for iOS and pay 30% of 0$. That seems wrong... Or maybe I would have have to track users installs and use time? That seems wrong...