I don’t have all the answers, but they can be found out with some reading of the Steemit white paper.
The creators and investors were given STEEM at the start, but they cannot just grant themselves more.
For changes, or ‘forks’, there has to be a consensus with the top people running the steemit blockchain witness servers. These folks are majority elected by Steemit users. Every user has up to 20 votes they can allocate, one per witness. Steemit was created to be self-policed by the users of it. If any user gets out of line, there is community pushback.
Most people that choose to sell use either Poloniex or Bittrex exchanges. It is much like the stock exchanges, the value fluctuates based off the last sale price and other people are buying when you sell.
As for answers to the rest, you can search on Steemit or get technical with it by reading the Steemit white paper.