About where the money comes from: New Steem is created everytime miners create a new block. Unlike Bitcoin where the rewards for a block go to the miners, Steem distributes part of the reward to people who post or upvote content.

About whether Steem is a scam: The creators of Steem own a large portion of the total supply of Steem. The developers could simply be slowly cashing out their Steem, or they could sincerely want to improve the platform.