How does Opera 50mb internet privilege works

Opera mini this days actually give users the preveledges to surf the internet using their app.

I understand, they partnered with local ISP but what I couldn’t comprehend is the level of the partnership, who pays who and what is the advantage of this partnership to both party?
Then what Technology does Opera deploy to monitor users traffic such that it doesn’t exceeded the 50mb.
And finally, how can a Developer intergrate thesame system into their browsers i.e free internet access to users…
even at the expense of the partnership.

Any response pls.