I’m not sure I’m understanding this so forgive me if I’m ignorant here, but is it not based off of their prices in fiat? 

Like 1 XMR = ~0.0023 BTC, and it’s exchanging at that rate because 1 XMR = ~$146 USD, which also is = ~0.0023 BTC. So you get that amount of Bitcoin /because/ that’s how much the XMR would be worth in fiat. Or is there another way to calculate it?