I wasn't sure where to post this question, so mods please feel free to move it.

Today I made an award reservation, where I took my amex mr moved them into my FF account and then booked a trip on a partner airline. I got a nice value of about 8 cents per point, which I am quite happy about.

It got me thinking, at the end of the day, who is the one that ends up paying for this?
Is it Amex, where the points came from in the first place?
Is it the airline I used to make the reservation?
Or is it the partner airline, that I will actually fly on?

I assume that its not a split cost, that would make things pretty messy for everyone to figure out. But how does it work? Anyone able to offer insight? Thanks