Chairs have almost complete freedom in running their conference. They can add and delete TPC members, change submission deadlines, add tracks and change review questions, for example.
For their own papers, a chair is treated just like any author, e.g., they cannot see reviewer identities of papers they have co-authored and cannot see review and scores until they have been made available to authors. However, if a chair wants to "cheat", there are probably ways to find out information, e.g., through some of the paper listing functions. The feature is mostly meant to prevent accidental disclosure of information, not willful snooping.
You can set up conferences as master and subconferences, the chair of the masterconference is treated as if he or she was a chair of the subconference as well.