Where do the rewards for posts come from?

Hey, I'm new to steem, kinda confused here. Where do the rewards for posts come from?

Clearly they don't seem to come from the people upvoting, because those people get rewards for curating. I can't imagine they come from the website itself, because where do they get the supply of SD to constantly reward people. Perhaps they come from block rewards? But then I'd imagine the currency would be quickly inflating, and it'd be difficult to peg against the dollar. Also, how is the total reward amount decided? Is it some amount per upvote? Or what?

Thanks for answering my question.