Each chain is run by that chain's committee of validators. This committee owns a key that is split between all of its validators. Each key share is useless on its own, but a collective signature gives validators full control over the chain.
ISC does not define how do you select validators to form a committee: it could be a solitary choice of the chain's owner, or it could be a public competition between candidates. ISC does not define how validators are rewarded, either.